© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
A comparison of fish bycatch communities between areas open and closed to prawn trawling in an Australian tropical fishery
a CSIRO Marine Research PO Box 120, Cleveland, QLD 4163, Australia
b CSIRO Mathematical Information and Statistics PO Box 120, Cleveland, QLD 4163, Australia
*Correspondence to I. Stobutzki; World Fish Center, PO Box 500 GPO, 10670 Penang, Malaysia; tel.: +60-4-626-1606; fax: +60-4-626-5530. e-mail: i.stobutzki{at}cgiar.org.
The bycatch fish community was compared between areas open and closed to prawn trawling in Australia's Northern Prawn Fishery to investigate the impacts of the fishery. Two regions of a large (
6648 km2) closure were compared, with three areas in each region, one closed to trawling (Closed) and two open to trawling, one near the closure (Near) and one farther from the closure (Far). Sampling was undertaken both day and night. The two regions and two times were analysed separately using both multivariate and univariate analyses to examine changes in overall community structure and differences in individual species. Overall the results were equivocal with respect to the impact of trawling. The multivariate and univariate analyses showed that in both regions, during both day and night, the bycatch fish community of the Far open area differed from the Near and Closed areas, while the latter were similar. This at least partly reflected differences in depth and sediment. For individual species, most showed no significant difference between the areas open and closed to trawling. Of the significant results there was no consistent tendency for species to be more likely to occur inside the closure or be at a higher density or larger size within the closure. Benthic and demersal species, those more susceptible to capture by prawn trawls, were not consistently less likely to occur or at a lower biomass in the open areas. The lack of a strong contrast in the fish community between the open and closed areas is probably due to the comparatively low effort in the fishery, the highly aggregated nature of the trawling and the fact the fishery does not target the bycatch species. These factors may reduce the potential impact of trawling on the fish bycatch.
Keywords: bycatch, closures, fish, fishing impacts, impacts of fishing, prawn trawling, shrimp trawling
Received 26 February 2001; accepted 5 May 2003.
 |
Introduction |
|---|
 Top Introduction MethodsResultsDiscussionReferences |
|---|
In recent years there has been increasing concern over the impact of fishing on continental shelf ecosystems (e.g. Jennings and Kaiser, 1998; Hall, 1999; Gislason et al., 2000). Fishing has both direct (e.g. removal of species) and indirect (e.g. habitat modification, changes in prey or predator densities) impacts on ecosystems. One of the most visible direct impacts of fishing is the capture of non-target species, known collectively as bycatch. Bycatch includes species that are unwanted and discarded (Discards) and species that are retained and sold (Byproduct).
The bycatch issue is of particular concern in prawn (shrimp) trawl fisheries. In most, the weight of bycatch is greater than the weight of commercial prawns. Prawn trawl fisheries globally are estimated to produce one-third of the total fishery discards (Alverson et al., 1994). The bycatch of tropical prawn trawl fisheries is dominated by fishes (Hall, 1999) that have a low survival after capture, even if discarded (Wassenberg and Hill, 1989; Hill and Wassenberg, 1990). The large volume of bycatch has contributed to the widespread belief that prawn trawling causes numerous and detrimental changes to ecosystems. However, the impact of prawn trawling on fish communities is generally unknown. The research that has been conducted has focused on species of commercial or recreational importance (e.g. Broadhurst and Kennelly, 1994; Nance and Scott-Denton, 1996). There have been few attempts to examine the impact of prawn trawling on the whole suite of bycatch species.
The impacts of fishing on target species are well documented, with changes seen in biomass, species diversity and size (Bianchi et al., 2000). The impacts of fisheries on bycatch species are less well-known, although some studies have documented changes in specific bycatch species (e.g. Casey and Myers, 1998; Pope et al., 2000). Where long-term catch data are not available, a comparison of areas open and closed to fishing is an alternative way to examine the impacts of fishing on communities and ecosystems. This type of comparison has been undertaken in many areas where marine reserves have been implemented to protect species or areas from fishing impacts (Alcala and Russ, 1990; Polunin and Roberts, 1993; Roberts, 1994; Rakitin and Kramer, 1996). We examined the impact of prawn trawling in Australia's Northern Prawn Fishery (NPF) on the fish (teleost and elasmobranch) bycatch by comparing areas open and closed to trawling. The specific hypotheses tested were:
- Ho1 Trawling does not result in an overall change in the bycatch community. Differences in the productivity of species and their susceptibility to trawling may result in different bycatch communities in trawled areas compared to non-trawled areas. In general, fishing is expected to impact the diversity of fish communities, examples have been shown in The North Sea, Georges Bank and Gulf of Thailand (see review by Hall, 1999). Given this, we may expect the areas closed to trawling to differ in community structure.
- Ho2 Trawling does not change the biomass or average size of bycatch species. There is evidence to suggest that fishing results in a reduction in the number of large species and large individuals (e.g. Rice and Gislason, 1996; Bianchi et al., 2000). Given this, we would also expect individual species that are impacted by trawling to have higher catch rates and a larger mean size in areas closed to trawling.
- Ho3 The impact seen on individual species does not vary with their susceptibility to trawling. Benthic and demersal species are more susceptible to capture by prawn trawls than pelagic species, as the trawls fish close to the sea floor with a low headrope height. Therefore, a higher proportion of benthic and demersal species would be expected to show significant reductions in abundance in areas open to trawling, than pelagic species.
- Ho2 Trawling does not change the biomass or average size of bycatch species. There is evidence to suggest that fishing results in a reduction in the number of large species and large individuals (e.g. Rice and Gislason, 1996; Bianchi et al., 2000). Given this, we would also expect individual species that are impacted by trawling to have higher catch rates and a larger mean size in areas closed to trawling.
Previous studies also suggest that certain species are likely to change in abundance in response to trawling. On Australia's North West Shelf, Sainsbury (1988) showed a reduction in the species targeted by fish trawling (lethrinids and lutjanids), while saurids and nemipterids increased in abundance. Longhurst and Pauly (1987) also suggest that saurids may benefit from trawling in the Gulf of Thailand. Harris and Poiner (1991) examined the differences in the fish community in a region of the NPF, after 20 years of prawn trawling. They found most species showed no change and of those that changed, around half had increased in abundance while other half decreased, the monacanthids were one group that appeared to have shown a large decrease. We examine our results to see whether species show the patterns expected based on this previous work.
|
Methods |
|---|
|
TopIntroductionMethodsResultsDiscussionReferences |
|---|
Study area
Our study focused on Australia's NPF, one of Australia's most valuable fisheries with 130 vessels taking 8500 t of prawns in 1999 (Sharp et al., 2000). The fishery started in the 1960s and the current managed area of the fishery covers over 6000 km of coastline and over 1 000 000 km2 of ocean (Figure 1). The vessels tow a twin gear configuration, usually with Florida Flyer trawl nets. The fishery is currently open for about 6 months of the year from April to June and then September to November. For majority of the season (approximately 25 weeks) the boats fish at night targeting tiger prawns (Penaeus semisulcatus and Penaeus esculentus) and endeavour prawns (Metapenaeus endeavouri and Metapenaeus ensis) (McLoughlin et al., 1997). The trawls are 3–4 h duration and the levels of bycatch high (Stobutzki et al., 2001).
|
The NPF includes several large areas that are permanently closed to trawling. Our study focused on the closure to the west of Groote Eylandt (Figure 1), which has been permanently closed to trawling since 1983. This closure is over 6648 km2 in area. Prior to closure, this area was trawled (1970–1982 average yearly fishing effort was 528 days). This closure was instigated to protect juvenile tiger prawns and their nursery habitats (Taylor, 1994). Our comparison focused on this closure because it contained areas that were trawled prior to the closure, increasing the likelihood that the environment would be similar to areas currently trawled. Most other permanent closures in the NPF cover shallow seagrass habitat (Taylor, 1994) that is likely to differ markedly from the main trawling grounds. In order to separate out possible trawling impacts, factors such as depth and bottom type of the open and closed areas should be as similar as possible. Depth, sediment type and acoustic measures of bottom roughness and hardness were recorded in order to factor out their influence on any observed patterns in the fish community.
The large size of the closure (over 6648 km2) also means it provides substantial refuge to the fish communities from trawling. The fish bycatch community is highly diverse, containing over 400 teleost species and 56 elasmobranch species (Stobutzki et al., 2001). These species range in size (7–300 cm) and also mobility, from demersal, sedentary or site-attached species (e.g. callionymids, diodontids, tetraodontids) to primarily pelagic species (e.g. carangids, engraulids, sphyraenids). The mobility of some of the fish species may result in them moving between the closure and trawled areas, diluting any effect of the closure. However, if trawling is having a significant effect on the community, the large size of the closure should provide significant refuge to detect differences.
Survey design
A pilot study was conducted in 1997 (Stobutzki et al., 2000) and the results from this were used to design the current study. In October 1998 sampling was undertaken using the 66 m "RV Southern Surveyor". A single 14 fathom (26.5 m) Florida Flyer demersal prawn trawl was used. The body of the trawl was made of 57 mm stretched mesh with a 150x150 mesh codend of 45 mm stretched mesh. The net was rigged with 100 m bridles and No. 9 Bison otter boards (490 kg). The net is the same as those used by commercial trawlers, although the latter tow two nets.
Two regions ("North Groote", "South Groote") of the closure west of Groote Eylandt (Figure 1) were sampled. In each region three areas were sampled during both day and night: an area closed to trawling (Closed); an area open to trawling adjacent to the closure boundary (Near); and an area open to trawling further from the closure (Far) (Figure 2). In each area (Closed, Near and Far) three 6x6 n. mile grids were sampled (Figure 2).
|
The North Groote region was sampled for 12 nights and 9 days followed by the South Groote region for 12 nights and 9 days. As the time of month may contribute significant variation to the catch rates (Salini et al., 2001) sampling was blocked with respect to this factor. In each region the sampling time was broken into blocks of 3 days/nights. During each block, each area (Closed, Near and Far) was visited for 1 day/night. The sampling was also blocked with respect to the time of day/night. The time of day/night was broken into three time periods, the first three trawls, the second three and the third three trawls. These did not always correspond to distinct times of day/night, due to logistic constraints. Sampling in each time period was conducted in one 6x6 n. mile grid. Trawls were aimed to be 0.5 h in duration.
The aim was to distribute the sampling randomly with respect to both blocking factors (block of days/nights and block of time of day/night), using Latin Square designs (Sokal and Rohlf, 1996). However, due to logistic constraints the design was modified and consisted of sampling the areas and the grids within areas as shown in Table 1. We aimed to carry out three trawls within each grid each day/night and this was accomplished unless there was gear failure. Table 2 provides a glossary to the terms used in the analysis, for reference.
|
|
The total catch of each trawl was weighed and the number and weight of large species (e.g. elasmobranchs) recorded. The remainder of the catch was then either sorted entirely or a subsample was sorted to determine species composition. The size of the subsample varied with the catch weight. The average subsample was 26% (±1.0% standard error (s.e.)) of catch weight (excluding the weight of species not subsampled). Individuals were identified to the lowest possible taxonomic level, which was species level for most. The weight (to the nearest 0.1 g) and number of individuals of each species were recorded. We also recorded the standard length of up to 20 randomly selected individuals of each species from each trawl. The catch rates were standardised by the duration of the trawl to the number of individuals per hour (n h–1) and weight per hour (kg h–1).
The depth of each trawl was recorded by automatic dataloggers on the vessel. Acoustic measures of the seabed were collected every 2 s, using an echo integrator connected to the output of the echosounder (RoxAnn). This provided measures of the seabed character, usually referred to as the "roughness" and "hardness" of the seabed. Spurious data, due to weather conditions or other factors, were identified and removed. This resulted in roughness and hardness values not being available for all trawls. The RoxAnn data for each individual trawl were then summarised as mean roughness and hardness and included in the analyses. Units are not provided for roughness and hardness as they are relative measures.
Sediment samples were collected in both regions using a 0.1 m2 Smith–MacIntyre grab from which a subsample of sediment was frozen. A total of 261 sediment grabs were collected across the grids. At each sediment station three replicate grabs were taken in close proximity. For each sediment sample a representative subsample was dispersed in 1% calgon solution and washed through a 0.063 mesh to separate the mud component. The gravel and sand fractions were then mechanically sieved through a standard Endecott sieve nest of 2.0, 1.0, 0.5, 0.25, 0.125, 0.063 mm mesh. The percentage mud in each sample (particle size <0.063 mm) was used in the analysis. An estimate of the percent mud in the sediment in the area of each trawl was taken from the nearest sediment sample.
Data analysis
Preliminary examination of the data showed significant region and day/night differences, as these were not the primary interest of the study the data were separated for analysis. Therefore, the data from each region (North Groote and South Groote) and each time (day and night) were analysed separately.
Ho1 Trawling does not result in an overall change in the bycatch community
The overall composition of the bycatch communities was examined using ordinations, performed on the transformed catch rates (log (n h–1+minimum n h–1) of species that occurred in more than 5% of trawls. Rare species were excluded from the analysis, as they were likely to be rare due to the prawn trawl's poor ability to capture them. The association matrix for trawls over species catch rates was formed using the Bray Curtis metric and principal coordinate analysis (classical scaling, Cox and Cox, 1994) was used to map the trawls into a small number of dimensions. Preliminary analyses suggested that the more robust technique of nonmetric multidimensional scaling gave similar orderings to the metric (classical) scaling mentioned above. Hence, the latter was used as the proportion of variance explained by using a few dimensions could be calculated (Cox and Cox, 1994).
More structured analyses to compare trawls, such as multivariate analysis of variance, or permutation tests, were not carried out in this study. This was due to the number of species exceeding the number of trawls, restricting the first type of analysis just mentioned and the complexity of the sampling design restricting the second type of analysis.
To examine whether the ordination patterns corresponded with changes in abiotic measures (depth, roughness, hardness and percent mud) or with commercial prawn trawling effort levels, Pearson's correlations (Sokal and Rohlf, 1996) were calculated between these and the principal components from the ordination.
A measure of the historical NPF commercial prawn trawling effort in the area of each trawl was obtained from commercial logbook data (held by the Australian Fisheries Management Authority), which provide a yearly value of the number of days trawled in each 6x6 n. mile grid. The effort for each grid was calculated as the number of days trawled in a grid between 1987 and 1996. The effort data since 1987 were used, as this year saw the start of substantial changes in the fishing effort and fleet characteristics (Robins and Sachse, 1994).
Ho2 Trawling does not change the biomass or average size of bycatch species
Two types of analysis were performed to determine whether individual species showed significant differences between open and closed areas. The first examined the probability of occurrence and the second, given a species occurred, the size of the biomass. These corresponding to the two components of the delta-lognormal distribution (Ortiz et al., 2000) assumed for biomass. This distribution was appropriate, as many species had a high proportion of zero biomass records. The first type of analysis was a logistic regression in SAS GENMOD (SAS Institute, 1997) for the probability of a species being present, with the effect of trawling as a factor to be tested. The second type of analysis looked at the positive biomass records using an SAS GLM (SAS Institute, 1997) approach to test the effect of trawling. In this analysis blocks of days/nights were taken into account and between block and within block variances estimated.
In both of the above types of analysis the potential influence of the covariates (depth, percent mud and commercial effort) was taken into account before the treatment effects were assessed. Prior to these analyses the correlations between the covariates were examined, to ensure none were strongly correlated. Roughness and hardness were not used as covariates as they were correlated with other factors and were not available for all trawls.
The GLM that examined differences in the positive biomass of species included the factor area, which had three levels (Closed, Near and Far). The patterns seen in the ordinations (Ho1) suggested that two specific contrasts between the areas should be investigated. The first contrast compared the Far area with the combined Closed and Near and the second contrast compared the Closed and Near areas. The effect of the block of sample days/nights and the block of time of day/night were also included. The block of sample days/nights had four levels in the night analyses and three levels in the day analyses (Table 1). The block of time of days/nights had three levels. The interactions between the area contrasts and the time were also examined. The effect of grid within area was included to partition variation due to this factor. Due to the unbalanced nature of the design the appropriate error terms to test the various effects were formulated from the table of expected mean squares.
The lengths of fish species were compared among the Far, Near and Closed areas. Analyses were run only for species where 10 or more individuals had been measured from all three areas within a region for day or night. The length frequency data of each species were examined for skewness and kurtosis and appeared normal, so the data were analysed without transformation. A one-way ANOVA examined whether the average length of a species was significantly different among areas. The ANOVA was performed separately for each region and time (day or night) combination using PROC GLM (SAS Institute, 1997). The analysis was weighted by the sample size as this varied among the areas. Where there was a significant effect of area a posteriori comparisons between the least squares means were used to determine which areas were significantly different.
Ho3 The impact seen on individual species does not vary with their susceptibility to trawling
The position of species in the water column affects their susceptibility to capture by prawn trawls. Prawn trawls fish close to the seafloor so species that occur in this area are more susceptible. Greater impacts would be expected in species that were most susceptible to capture. Therefore, the results of the univariate analyses were examined in relation to position of species in the water column. Species were classified (based on Froese and Pauly, 2002) as benthic (species that are known to rest on the sea floor), demersal (species that live in the water column just above the sea floor), benthopelagic (species that move through the water column, sometimes near the sea floor and sometimes in the upper water column) and pelagic (species that are primarily in the upper water column). The percentage of significant contrasts between the areas was examined within each type.
Previous research into trawling impacts on fish species suggests that some groups of species show significant changes, particularly in the families Lethrinidae, Lutjanidae, Monacanthidae, Nemipteridae, Bathysauridae and Synodonidae (Sainsbury, 1988; Harris and Poiner, 1991). The results for species within these families were examined to see whether they followed the patterns expected from previous studies.
|
Results |
|---|
|
TopIntroductionMethodsResultsDiscussionReferences |
|---|
There was little difference (2–3 m) in average water depth between North Groote and South Groote but in both regions the Far area was around 30 m deep compared to 20–22 m for the Closed and Near (Figure 3a). The percentage of mud was also higher in these deeper Far areas (Figure 3b). This distribution of fine sediment did not correspond to the roughness of the seabed, which was highest in the Closed area (Figure 3c). Hardness of the seafloor, tended to be higher in the Far areas in each region (Figure 3d). Historical fishing effort shows considerable difference between the regions. Effort at the Far area in North Groote was much higher than at the Near site, but this contrast was not evident at South Groote (Figure 3e).
|
, North Groote night; 
, North Groote day; •, South Groote night; 
, South Groote day).Ho1 Trawling does not result in an overall change in the bycatch community
The four ordinations, each region at two times, showed consistent results. The first three dimensions from the ordinations explained between 45 and 55% of the variation (Table 3). In both regions, at both day and night the ordinations showed separation between the Far area and the combined Near and Closed areas (Figure 4). This separation of the Far area occurred on the second dimension in all ordinations except North Groote at night, where the separation occurred on a combination of the first and second dimensions (Figure 4).
|
, Near area open to trawling; 
, Closed to trawling). The groups labelled refer to the structure within the combined Closed and Near sites.
|
In the four ordinations the Near and Closed sites were mixed, but showed distinct structure, separating into two groups on the first principal coordinate (Figure 4). These ordination groups do not reflect the grouping of the sites spatially. However, in South Groote the groupings of the Closed and Near sites appear to reflect an effect of the block of days/nights sampled (Figure 5). The sites sampled in the first two blocks of days/nights grouped together and those sampled in the last two blocks of days/nights grouped together, separating on the first principal coordinate. The first two blocks of days/nights sampled in South Groote coincided with a full moon. In contrast, the grouping of Closed and Near sites at North Groote does not reflect any effect of the block of days/nights sampled (Figure 5). The sampling at North Groote took place around the waxing moon. There was no clear effect of the time of day/night sampled in any ordination.
|
All ordinations showed significant correlations between the abiotic measurements and the dimensions and there was some consistency among the regions and times (Table 4). This suggests that the grouping of sites in the ordinations corresponded to some extent changes in the abiotic measures. Overall there were fewer significant correlations with the first dimension. The exception was North Groote at night, which showed strong correlations with depth, hardness and effort. For North Groote during the day and South Groote at both times, the second dimension, on which the Far area separated from Near and Closed, had the strongest correlations. At South Groote at both times, depth and percent mud had strong positive correlations and roughness negative correlations. At North Groote during the day, the strong correlations were with roughness, hardness and effort, while during the night, with effort, percent mud and depth.
|
Ho2 Trawling does not change the biomass or average size of bycatch species
Most of species did not show significant differences in their probability of occurrence between areas (Table 5). Between 81 and 89% of species showed no difference in their probability of occurrence between the Closed and Near areas in either region, at day or night. Between 74 and 85% of species showed no difference between the combined Closed and Near areas and the Far area, in either region, at day or night. The covariate of depth was significant for more species than covariates effort and mud in both regions at night (Table 5). However, during the day both depth and mud were significant for a similar number of species (Table 5).
|
Of the species that did show a significant difference between the Closed and Near areas, in North Groote at both night and day and in South Groote at night, most species had a higher probability of occurrence in the Near area (Table 5). In contrast, at South Groote during the day most species had a higher probability of occurrence in the Closed area (Table 5). Of the species that had a significant difference between the combined Closed and Near areas and the Far area, at North Groote during both day and night, most species had the highest probability of occurrence in the combined Closed and Near areas (Table 5). In comparison, at South Groote during both day and night, most species had the highest probability of occurrence in the Far area (Table 5). Of the species that did show significant differences in their probability of occurrence, few had both contrasts significant in the one area either day or night (Table 5).
Most species did not show significant contrasts for the comparison of biomass amongst the areas, 89–94% of species showed no significant contrast between the Closed and Near areas and 85–99% for the contrast between the combined Closed and Near areas and the Far area (Table 6). Similar to the analysis of probability of occurrence, depth was a significant covariate for the highest number of species at night (Table 6). At South Groote the covariate of Block of nights/days was significant for a high proportion of species (Table 6), confirming the pattern seen in the ordination (Figure 5).
|
Of the species that did show a significant difference in biomass between the Closed and Near areas, in North Groote night and day and in South Groote day, most species had a higher biomass in the Near area (Table 6). In contrast, at South Groote night most species had a higher biomass in the Closed area (Table 6). Of the species that had a significant difference between the combined Closed and Near areas and the Far area, at North Groote both day and night and South Groote at night most species had the highest biomass in the combined Closed and Near areas (Table 6). In comparison, at South Groote day, most species had the highest biomass in the Far area (Table 6). Of the species that did show significant differences in their biomass, few had both contrasts significant in the one area either day or night (Table 6).
The results for individual species were rarely consistent between times or regions for either the analysis of their probability of occurrence or their biomass (Tables 7 and 8). More species had both contrasts (Closed vs Near area and combined Closed and Near areas vs Far area) significant at the one time (day or night) than showed consistency between day and night. There were fewer species that showed consistency between the two regions. Only one species Gerres macracanthus showed the same contrast significant for both probability of occurrence and biomass (Tables 7 and 8).
|
|
When the mean size of species was compared among areas within the regions they did not show larger individuals in the Closed areas (Table 9). Within a region, up to 39% of species had no significant difference among areas and 43–77% were largest in the Far or Near areas, compared to 0–6% in the Closed area (Table 9).
|
Ho3 The impact seen on individual species does not vary with their susceptibility to trawling
The demersal and benthic species, which would be expected to be more vulnerable to trawling, did not show more significant comparisons (Table 10). There was not much difference between the groups in terms of the percentage of significant comparisons.
|
Table 11 summarises the results for the taxa identified by previous studies as showing changes in response to trawling. Most of the comparisons were not significant. The saurids (family Bathysauridae and Synodontidae) and nemipterids, when a significant result was found, did tend to be more likely to occur or have higher biomass in the areas open to trawling than the closed. The lethrinids, lujanids and monacanthids showed very few significant results (Table 11).
|
|
Discussion |
|---|
|
TopIntroductionMethodsResultsDiscussionReferences |
|---|
Overall the results from our comparison of areas open and closed to prawn trawling do not show the patterns expected due to strong fishing impacts on the bycatch species.
Ho1 Trawling does not result in an overall change in the bycatch community
In both regions the bycatch community of the Far area was clearly different. However, the community in the Near area, also open to trawling, was not different from the Closed. The contribution of fishing to this pattern is unclear. In North Groote the levels of commercial fishing effort in the Far area (Figure 2) are greater than the near. Therefore, we expect the Far area to differ more from the Closed than the Near. However, in South Groote the effort levels are similar in Near and Far (Figure 2) and yet the overall pattern was the same as North Groote. The differences in the bycatch community in the Far areas were also contributed to by differences in depth and bottom characteristics. The Near areas, which were similar in depth and bottom characteristics to the Closed areas but differed in terms of being open to fishing had an overall community similar to the Closed areas.
Ho2 Trawling does not change the biomass or average size of bycatch species
There was no clear reduction in overall biomass or size of species in the open areas. Most species were just as likely to be found in the open areas as the closed and when found had similar biomasses. The significant differences seen were not consistent between night and day or regions. Of the species that displayed differences in size, most were larger in the open areas. The lack of significant differences is not likely to be due to a lack of low power of the analysis. We could not directly estimate the power of the analysis. Power analysis theory has been developed for repeated measures designs, one-way ANOVAs or higher factorials (Peterman, 1990; Muller et al., 1992; O'Brien and Muller, 1993), however, our analysis had an incomplete block design structure in conjunction with nested treatments for which the power theory has not been developed. Examining the significant differences detected gives some indication of the power. Some of the significant differences in probability of occurrence or biomass were small (3% difference between areas) indicating the analysis had reasonable power.
Ho3 The impact seen on individual species does not vary with their susceptibility to trawling
As expected demersal and benthic species, those most susceptible to capture by prawn trawling, showed the most significant differences among the areas in terms of their probability of occurrence (Table 10). However, the difference was not great and there was little difference in the proportion of species that showed a significant difference in terms of biomass. Therefore, there was no clear link between the susceptibility of species and the differences between open and closed areas.
The results for the species highlighted by previous studies also did not show strong differences between the areas open and closed to trawling. Again the majority of results were not significant. As expected where significant, the saurids and nemipterids tend to more likely occur at higher biomass in the areas open to trawling, however, this result was not consistent between the two regions and times. The species that were expected to show a decrease, lutjanids, lethrinids and monacanthids, showed non-significant results for most comparisons.
Overall the bycatch community in this fishery, while showing some differences between open and closed areas, does not show the strong changes in community structure or species' biomass or size that might be attributed to the impacts of fishing. The differences that were apparent were also contributed to by differences in depth and bottom characteristics, not clearly attributable to impacts of fishing. These results agree with previous research in tropical Australian prawn trawl fisheries. Harris and Poiner (1991) examined the catch rate of fish species in one area of the NPF before and after 20 years of trawling. They found no difference for 63% of the species. Pitcher et al. (2000) compared areas open and closed to trawling in the northern Great Barrier Reef (GBR), Australia. They found that more fish species showed significant differences in abundance across the shelf, rather than between the open and closed areas. There are also examples of benthic invertebrate communities between trawled and untrawled areas in the North Sea where no clear differences between the open and closed areas have been detected (Hall et al., 1993). There are, however, numerous examples of trawl fisheries that have affected community structure (e.g. Engel and Kvitek, 1998; Jennings and Kaiser, 1998). The results from our study are likely to be due to characteristics of the fishery such as the level and pattern of effort in the fishery and the fact the bycatch species are not targeted and characteristics of the bycatch species.
The NPF has only 130 boats with fishing effort of approximately 2000 days per year (Sharp et al., 2000). This is a relatively low level of effort when compared to fisheries in similar regions, such as the Queensland East Coast, which had over 800 prawn trawlers fishing 106 000 days in 1999 (Williams and Dredge, 2002). In conjunction with these low effort levels, trawling also tends to be aggregated both spatially and temporally. Trawling is highly aggregated within the fishery, with <25% of the managed area trawled, and of this most relatively lightly. Only about 25% of the trawled grids have high effort levels (>1000 h yr–1) (Stobutzki and Pitcher, 1999). At a finer spatial scale, within the 6 n. mile grids used to report effort, trawls are not spread evenly. Trawlers repeatedly trawl the same track and large areas of the grids are not trawled (Stobutzki and Pitcher, 1999). This aggregation means that even within high-effort grids, substantial areas may be only lightly trawled or untrawled. This aggregation results in substantial variation in the level of impact on bycatch species and also possible spatial refuges for species within the trawl grounds. Aggregated trawling is likely to have less effect on species than if it were evenly spread across the area (Pitcher et al., 2000). The bycatch species also have a temporal refuge from trawling, due to the management closure of the fishery for approximately 6 months of the year. These spatial and temporal refuges and the low overall effort in the fishery are likely to reduce the impact of fishing on the bycatch species.
The fact the fishery is not targeting the bycatch species, may also reduce the impacts of fishing. The fishery has been shown to be impacting the stocks of the target prawn species, with overfishing a concern (NPF FAG, 2000) but this may not be the case for the bycatch. Many of the demonstrations for the effect of closures have been based on fisheries that are selective and targeting particular species (e.g. Roberts, 1994; Rakitin and Kramer, 1996). These closures often show an increase in size and abundance within the closure, after the removal of fishing pressure. With respect to bycatch in the NPF, we were examining the impacts on species that are not targeted and where the fishing method (prawn trawling) is not likely to be the most effective method for catching most species. Therefore the impact of the fishery on these species is likely to be less than on the target species.
The mobility of fish bycatch species may also reduce the impact of trawling or the contrast between the areas. The bycatch species range from small, relatively site-attached species, to more mobile species. The mobility of some species would allow them to readily move across the closure boundaries potentially diluting the protective effect of the closure. The large size of the closure studied here (6648 km2) means that it may be effective for even some of the more mobile species. It may be that a greater impact would be seen in the sessile communities, but these have yet to be evaluated.
The high natural variation in these communities may also obscure any effect of fishing. Stobutzki et al. (2001) showed significant variation in bycatch communities among different fishing regions of the NPF. If the contrast between the open and closed areas is comparatively small and the natural variation high it would be difficult to detect an impact due to fishing. The fact that this is a tropical system may also influence the extent of the effects of fishing. Bianchi et al. (2000) examined the impacts of fishing on the size spectra of communities and found the results were less conclusive in tropical systems, suggesting the species may be less sensitive.
Time-series data would provide a validation of the lack of contrast seen in this comparison. However, as previously mentioned this is not available for this fishery, except for one location over three points in time (Harris and Poiner, 1991). This restricted time series showed similar results, with most species showing no change. While fishing is known to impact species and ecosystems (Bianchi et al., 2000), our comparison of areas open and closed to prawn trawling did not demonstrate fundamental changes in the bycatch community of the fishery. There were no clear shifts in community structure or trends shown by species in terms of biomass or size. The nature of this fishery with long seasonal closures, comparatively restricted, highly targeted and aggregated effort, may reduce the impact on the fish bycatch.
|
Acknowledgements |
|---|
We thank the commercial fishers who took the time to record their catch and support this project; the Northern Prawn Fishery Management Advisory Committee for its support; J. Bishop and M. Haywood for preparing the commercial effort data; Q. Dell and D. Vance for cleaning the RoxAnn data; the fishing, scientific and electronics crew of the RV Southern Surveyor who made all the field work possible; S. Hall, B. Hill, D. Brewer, D. Milton, D. Heales, J. Salini, G. Fry, S. Blaber and two anonymous referees for constructive comments on the manuscript. This project was funded by the Australian Fisheries Research and Development Corporation (FRDC 96/257) and CSIRO Marine Research.
|
References |
|---|
|
TopIntroductionMethodsResultsDiscussionReferences |
|---|
-
Alcala A.C. and Russ G.R. (1990) A direct test of the effects of protective management on abundance and yield of tropical marine resources. Journal du Conseil International pour l'Exporation de la Mer 46:40–47.
Alverson D. L., Freeber M. H., Murawski S. A., Pope J. G. (1994) A global assessment of fisheries bycatch and discards. FAO Fisheries Technical Report No. 339. 233 pp.
Bianchi G., Gislason H., Graham K., Hill L., Jin X., Koranteng K., Manickchand-Heileman S., Paya I., Sainsbury K., Sanchez F., Zwanenburg K. (2000) Impact of fishing on size composition and diversity of demersal fish communities. ICES Journal of Marine Science 57:558–571.
Broadhurst M.K. and Kennelly S.J. (1994) Reducing the by-catch of juvenile fish (mulloway Argyrosomus hololepidotus) using square-mesh panels in codends in the Hawkesbury river prawn-trawl fishery, Australia. Fisheries Research 19:321–331.[CrossRef][Web of Science]
Casey J.M. and Myers R.A. (1998) Near extinction of a large, widely distributed fish. Science 281:690–691.
Cox T.F. and Cox M.A.A. (1994) Multidimensional Scaling(Chapman and Hall, NY) 213 pp.
Engel J. and Kvitek R. (1998) Effects of otter trawling on a benthic community in Monterey Bay National Marine Sanctuary. Conservation Biology 12:1204–1214.[CrossRef][Web of Science]
FishBase (2002) World Wide Web electronic publication. (http://www.www.fishbase.org), accessed 02 January 2003.
Hall S.J. (1999) The Effects of Fishing on Marine Ecosystems and Communities(Blackwell, Oxford) 200 pp.
Hall S.J., Robertson M.R., Basford D.J., Heaney S.D. (1993) The possible effects of fishing disturbance in the northern North Sea: an analysis of spatial patterns in community structure around a wreck. Netherlands Journal of Sea Research 31:201–208.
Harris A.N. and Poiner I.R. (1991) Changes in species composition of demersal fish fauna of Southeast Gulf of Carpentaria, Australia, after 20 years of fishing. Marine Biology 111:503–519.[CrossRef]
Hill B.J. and Wassenberg T.J. (1990) Fate of discards from prawn trawlers in Torres Strait. Australian Journal of Marine and Freshwater Research 41:53–61.[CrossRef]
Jennings S. and Kaiser M.J. (1988) The effects of fishing on marine ecosystems. Advances in Marine Biology 34:201–352.
Longhurst A.R. and Pauly D. (1987) Ecology of Tropical Oceans(Academic Press, San Diego, CA) 407 pp.
McLoughlin K., Wallner B., Staples D. (1997) Fishery Status Reports 1996(Bureau of Resource Sciences, Canberra, Australia) 220 pp.
Muller K.E., LaVange L.M., Ramey S.L., Ramey C.T. (1992) Power calculations for general linear multivariate models including repeated measures applications. Journal of the American Statistical Association 87:1209–1226.[CrossRef][Web of Science]
Nance J.M. and Scott-Denton E. (1996) Bycatch in the Gulf of Mexico Shrimp Fishery. In Hancock D.A., Smith D.C., Grant A., Beumer J.P. (Eds.). Developing and Sustaining World Fisheries Resources: the State of Science and Management pp. 98–110 Second World Fisheries Congress Proceedings, Melbourne, Australia, CSIRO.
NPF FAG (Northern Prawn Fishery Fishery Assessment Group). (2000) Status of tiger prawn stocks at the end of 1999. Fishery Assessment Group Report to the Australian Fisheries Management Authority, Canberra. 8 pp.
O'Brien R.G. and Muller K.E. (1993) Unified power analysis for t-tests through multivariate hypotheses. In Edwards L.K. (Ed.). Applied Analysis of Variance in Behavioral Science(Marcel Dekker, Germany) pp. 297–344.
Ortiz M., Legault C.M., Ehrhardt N.M. (2000) An alternative method for estimating bycatch from the U.S. shrimp trawl fishery in the Gulf of Mexico, 1972–1995. Fisheries Bulletin 98:583–599.
Peterman R.M. (1990) Statistical power analysis can improve fisheries research and management. Canadian Journal of Fisheries and Aquatic Sciences 47:2–15.
Pitcher C.R., Poiner I.R., Burridge C.Y. (2000) The implications of the effects of trawling on sessile mega-zoobenthos on a tropical shelf in northeastern Australia. ICES Journal of Marine Science 57:1359–1368.
Polunin N.V.C. and Roberts C.M. (1993) Greater biomass and value of target coral-reef fishes in two small Caribbean marine reserves. Marine Ecology Progress Series 100:167–175.[Web of Science]
Pope J.G., MacDonald D.S., Dann N., Rynolds J.D., Jennings S. (2000) Gauging the impact of fishing mortality on non-target species. ICES Journal of Marine Science 57:689–696.
Rakitin A. and Kramer D.L. (1996) Effect of a marine reserve on the distribution of coral reef fishes in Barbados. Marine Ecology Progress Series 131:97–113.[Web of Science]
Rice J. and Gislason H. (1996) Patterns of change in the size spectra of numbers and diversity of the North Sea fish assemblage, as reflected in surveys and models. ICES Journal of Marine Science 53:1214–1225.
Roberts C.M. (1994) Rapid build-up of fish biomass in a Caribbean marine reserve. Conservation Biology 9:815–826.[CrossRef][Web of Science]
Robins C. and Sachse M. (1994) A creep called technology: evolution of the fleet. In Pownall P.C. (Ed.). Australia's Northern Prawn Fishery; the First 25 Years(Northern Prawn Fishery, Cleveland, Australia) pp. 23–34.
Sainsbury K.J. (1988) The ecological basis of multispecies fisheries management of a demersal fishery in tropical Australia. In Gulland J.A. (Ed.). Fish Population Dynamics(John Wiley, Chichester) pp. 349–382.
Salini J., Brewer D., Farmer M., Jones P. (2001) Lunar periodicity of prawns and bycatch in trawls from the Gulf of Carpentaria, northern Australia. Marine Biology 138:975–983.[CrossRef]
SAS Institute P. (1997) SAS/STAT Software; Changes and Enhancement through release 6.12(SAS Institute Inc, Cary, NC).
Sharp A., Bishop J., Harris D. (2000) Northern Prawn Fishery and Kimberley Prawn Fishery Data Summary 1999(Australian Fisheries Management Authority, Canberra) 46 pp.
Sokal R.R. and Rohlf F.J. (1996) Introduction to Biostatistics 2nd Edition (Freeman and Company, New York) 369 pp.
Stobutzki I.C. and Pitcher R. (1999) Assessing the response of bycatch communities to prawn trawling. In Buxton C.D. and Eayrs S.E. (Eds.). Establishing Meaningful Targets for Bycatch Reduction in Australian Fisheries(Australian Society for Fish Biology, Sydney, Australia) pp. 96–105 Australian Society for Fish Biology Workshop Proceedings, Hobart, September 1998.
Stobutzki I., Blaber S., Brewer D., Fry G., Heales D., Jones P., Miller M., Milton D., Salini J., Van der Velde T., Wang Y.-G., Wassenberg T., Dredge M., Courtney A., Chilcott A., Eayrs S. (2000) Ecological sustainability of bycatch and biodiversity in prawn trawl fisheries. Final Report to the Fisheries Research and Development Corporation (FRDC). Project 96/257, p. 512, FRDC, PO Box 222, Deakin West ACT 2600, Australia. 530 pp.
Stobutzki I.C., Miller M.J., Jones P., Salini J.P. (2001) Bycatch diversity and variation in a tropical Australian penaeid fishery; the implications for monitoring. Fisheries Research 53:283–301.[CrossRef][Web of Science]
Taylor B. (1994) A delicate balancing act: management of the fishery. In Pownall P.C. (Ed.). Australia's Northern Prawn Fishery; the First 25 Years(Northern Prawn Fishery, Cleveland, Australia) pp. 23–34.
Wassenberg T.J. and Hill B.J. (1989) The effect of trawling and subsequent handling on the survival rates of the by-catch of prawn trawlers in Moreton Bay, Australia. Fisheries Research 7:99–110.[CrossRef][Web of Science]
Williams L.E. and Dredge M.C.L. (2002) Trawl fisheries of Queensland. In Williams L.E. (Ed.). Queensland's Fisheries Resources: Current condition and Recent Trends 1988–2000(The State of Queensland, Department of Primary Industries, Australia) pp. 16–21 180 pp.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||