© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Age estimation, growth and maturity of the European hake (Merluccius merluccius (Linnaeus, 1758)) from Iberian Atlantic waters
Instituto Español de Oceanografía 36280 Vigo, Spain
*Correspondence to C. Piñeiro; tel: +34 86 492111; fax: +34 86 492351. e-mail: carmen.pineiro{at}vi.ieo.es; web: http://www.ieo.es.
Difficulties in age estimation for hake (Merluccius merluccius) have hampered the assessment of stocks. Here, we describe new, agreed ageing criteria based on the interpretation of the pattern of otolith growth. Improved estimates of von Bertalanffy growth parameters, and new estimates of maturity ogive parameters and length–weight relationships for European hake from Iberian Atlantic waters are presented. The results came from a study carried out during 1996–1997 and provide the first published account of the main life history traits of Southern stock hake. von Bertalanffy growth parameters of males were L
= 70cm, K = 0.18 year–1, and t0=–0.97 year, and those of females were L = 89cm, K = 0.13 year–1, and t0 = –1.15 year. Growth of sexes differed from age 3 onwards, with females being on average larger and heavier than males. The estimated total length (L, cm)–total weight (W, g) relationships were W=0.0132135L2.8134246 for males and W=0.0086471L2.942563 for females. Spawning took place from December to May with a peak in February. The mean length and age at first maturity were 32.8 cm at 2.5 years for males and 45 cm at 4.4 years for females.
Application of new ageing criteria showed that otolith sections may be used to determine ages up to 5 years in a consistent manner. These results indicate that hake of the Southern stock grow at higher rates and mature earlier than previously considered. Summaries of hake's life history parameters from other marine regions are also presented in order to make information that belongs largely to the grey literature available.
Keywords: age-at-maturity, growth, Iberian Atlantic waters, length-at-age, Merluccius merluccius, reproduction, southern stock, von Bertalanffy parameters
Received 17 July 2001; accepted 4 March 2002.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionConclusionsReferences |
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The European hake (Merluccius merluccius) is one of the most heavily exploited fish species in Western European demersal fisheries and is taken as part of mixed-species fisheries in the Northeast Atlantic (Casey and Pereiro, 1995). The assessment of hake in the area is undertaken annually by the Working Group of Southern Shelf Demersal Stocks (WGSSDS) of the International Council for the Exploration of the Sea (ICES). Despite the lack of a sound biological basis, since 1978 the WGSSDS distinguishes two hake stocks for assessment purposes: the Northern stock (ICES Division IIIa, Sub-areas IV, VI and VII and Divisions VIIIa-b) and the Southern stock (ICES Divisions VIIIc and IXa; Figure 1; ICES, 1979). A geographical barrier called Cap Breton Canyon separates these stocks.
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European hake fisheries off the Iberian Peninsula (Southern stock) have been operating for many years. Hake is of major importance due to its high market value to both Portuguese and Spanish fisheries. It is caught as target or as by-catch by a variety of vessels and gears, the fishery is exclusively for human consumption and hake is consumed fresh.
Over the past few years, the WGSSDS has been concerned with the status of European hake stocks, which are considered outside safe biological limits (ICES, 1999). Knowledge of hake's biological traits (especially growth and maturity) is considered insufficient to improve the assessment of stocks. For the Southern stock in particular, catch-at-age analyses have caused problems due to deficiencies in landing data and difficulties in age estimation (ICES, 1998). Since 1991, the latter problem has been overcome by converting length compositions of landings to age compositions using numerical methods (Kimura and Chikuni, 1987; ICES, 1994). Nevertheless, the annual assessment of Southern stock hake causes great scepticism, which emphasises the need to develop reliable ageing methods. In 1999, the WGSSDS made several changes supported by the results of two studies carried out under the auspices of the EU BIOSDEF (Anon., 1998) and DEMASSESS projects (Anon., 2000). These studies greatly enhanced knowledge of the biological parameters of hake, and improved age estimation procedures. One successful application of the latter was the use of empirical age-length keys (ALK) for the first time in the 1999 assessment of the Southern stock (ICES, 2000).
European hake age estimation relies on interpretation of the pattern of ring formation in otoliths. The unusual complexity of this task has been reported widely in the literature (Hickling, 1933; Bagenal, 1954; Meriel-Busy, 1966; Robles et al., 1975; Descamps and Labastie, 1978; Iglesias and Dery, 1981; Goñi, 1983; Goñi and Piñeiro, 1988; Guichet, 1988; Piñeiro and Hunt, 1989; Piñeiro and Pereiro, 1993; Morales-Nin et al., 1998), and several international workshops have been devoted to the development of a reliable ageing method for otoliths of this species (Anon., 1983, 1984, 1986). However, until recently these efforts had been fruitless. Two workshops on hake otolith interpretation conducted in 1997 and 1999, within the above-mentioned projects, developed standard ageing criteria to be adopted by age readers from institutions involved in hake stock assessment. The main achievement of these efforts was a set of internationally accepted interpretation criteria for hake otoliths up to age 5. Development of these ageing criteria was derived from previous research studies on the growth pattern of hake otoliths carried out by the authors of this work, who also led the above-mentioned workshops.
Despite the large number of available studies on the biology of European hake from the Northeast Atlantic, the majority of these have been published in national journals, collected in ICES contributions, or ICES working group and project reports, which are not widely distributed (Anon., 1998, 2002). Only in 2000, Lucio et al. published biological information of hake from the Bay of Biscay (Northern stock: ICES Divisions VIIIa,b,d). This paper presents improved estimates of the main biological characteristics of European hake from Iberian Atlantic waters (Southern stock) and for the first time includes growth information based on standardised otolith ageing criteria. It also constitutes the first published account of the life history parameters of hake from the Southern stock and includes a summary of the knowledge of hake's life history parameters obtained by different authors in the Northeast Atlantic, thus highlighting information that belongs largely to the grey literature.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionConclusionsReferences |
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Sampling
Hake specimens were collected from commercial landings during the BIOSDEF project (July 1996–June 1997) and from survey catches (September and October 1996) off the Northwest Iberian Peninsula. Sampling was done according to a random stratified design at quarterly intervals covering the length range of hake in the study area (ICES Divisions VIIIc and IXa, Figure 1). Total length (cm), total weight (g), sex and maturity state were recorded from all the specimens sampled. A summary of samples collected for this work is presented in Table 1.
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Age interpretation
The lack of agreement in the application of ageing criteria and procedures in hake otolith reading revealed the need to derive a set of internationally agreed ageing protocols, allowing otolith readers from different laboratories to age hake consistently. The otolith preparation and ageing methods used in this study were agreed in the two above-mentioned international workshops and rely heavily on our previous experience on hake ageing studies. The otolith terminology of Secor et al. (1995) has been used throughout this work.
Otoliths were sectioned on the dorso-ventral plane and thin sections mounted on glass slides using the technique described by Piñeiro et al. (1996). Two experienced readers aged the otoliths using a stereomicroscope under reflected light at x20 magnification. Readers were calibrated with a reference collection available at the Instituto Español de Oceanografía (IEO) prior to viewing the study material. A microscope-mounted video camera and monitor was used for discussion. Only otoliths with which the age estimates of the two readers coincided were retained for the growth study.
Hake otolith sections have a concentric pattern of translucent and opaque bands around the nucleus. One annulus or annual ring consisted of an opaque and of a translucent ring or band. The appearance of bands or rings varies along the dorsal–ventral axis of the sections, showing subsidiary rings within the main opaque and translucent pattern. Counts of winter rings that appeared as dark narrow bands (translucent), preferably on the ventral region, were used to estimate ages. A winter ring consisted of a single translucent band or of a series of two or three clustered translucent rings (caused by intermittent growth during this period). To locate the first winter ring previous knowledge was employed, in particular the modes of the size distributions from the surveys carried out annually since 1983 by the IEO (Piñeiro et al., 1992). These data show that the youngest fish enter the fishery during the third quarter with a modal size of 12–15 cm TL. This size corresponds to a radio distance of 1–1.5 mm from the otolith basis. Thus, in an average otolith the first winter ring may be found at around 1.3 mm from the nucleus. Additional information from discard surveys carried out in 1994 and in 1997–1999 indicates that the size of 0-age hake in December ranges between 17 and 20 cm (Pérez et al., 1996).
Hake growth during the first year of life is characterised by the occurrence of three checks or "false rings" that appear around the nucleus of the otolith. The existence of these "false rings" has been reported by several authors and they have been associated with: (1) the larval phase, (2) the pelagic phase, and (3) the onset of the demersal phase (Descamps and Labastie, 1978; Iglesias and Dery, 1981; Goñi and Piñeiro, 1988; Piñeiro and Pereiro, 1993). The first annual ring (winter ring) appears either after the so-called "demersal" check or may be co-incident with it (Piñeiro and Hunt, 1989). The varying position of this annual ring relative to the nucleus appears to be related to the extended spawning period of the species.
Growth during the first and second years is large by comparison with later increments. A well-marked check is frequently found on the otolith during this period (at around 1.5 mm from the nucleus). This check is not an annual ring but it may be co-incident with the first or with the second annual rings (Goñi and Piñeiro, 1988). It appears independently of the season, is linked to some still unknown biological or behavioural event, and is considered a reference mark when otolith reading is undertaken. The second annual ring (winter ring) appears after this check at around 2 mm from the nucleus although its position varies. The third annual ring appears afterwards and is usually preceded by another translucent ring.
Classification of the otolith edge type (translucent or opaque) tends to be complicated by the high incidence of false rings. Nevertheless, in this study we assumed that otolith edges followed the predominant pattern of translucent in winter and opaque in summer, and by convention an otolith with a translucent edge is not considered to be 1 year older until the 1st of January. Edge interpretation of the otolith sections in the study area was especially difficult for samples caught between late spring (May) and summer (July) and in otoliths from young fish. Very often sections presented faint rings at the edge and the mounting procedure did not permit checking the edge type on both faces, therefore whole otoliths were preferred for this matter. A length-stratified sub-sample of 350 whole otoliths of fish 8–40 cm long were selected to analyse the evolution of the marginal growth of otoliths through an annual cycle. Larger fish were not selected because the thickness of whole otoliths from older individuals hides the translucent bands. A selection of otolith sections (n=526) from hake of the same size range and collected in the same area of study, was used to compare the evolution of the otolith edge with both procedures.
Precision of readers was assessed by comparing the readings in a random sample of 100 otolith sections viewed independently by two readers. The average percentage of error (APE) was calculated following the method of Beamish and Fournier (1981).
Growth parameters
Age length keys (ALK) were produced for males, females and for the two sexes combined, and their mean lengths at age and standard deviations calculated. von Bertalanffy growth curves (von Bertalanffy, 1938) were fitted to the data applying the least-squares approach of the ® Microsoft Excel Solver routine using the Newton algorithm. This growth curve describes fish length as a function of age and its equation is as follows:
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is the asymptotic mean length, K is a rate constant that determines how fast Lt approaches L and t0 is the age at which the mean length is zero. Initially, the three parameters were estimated for both sexes and the two sexes combined without any restriction. Because the estimated value of L for the sexes combined was lower than the largest hake found in the study area, in a second trial we decided to adjust it to the largest size observed in the commercial landings with a 5% of increase to approximate the asymptotic mean length (Taylor, 1960, 1962; Pauly, 1985). Thus, to estimate K and t0, for the sexes combined, L was fixed at 120.5 cm.
Given that sets of growth parameters for European hake estimated by different authors were available, we decided to evaluate their reliability using the wide application phi-prime test (
') (Pauly and Munro, 1984). This test provides an indication of the reliability of age estimates since it has been suggested that phi-prime test values are similar for the same species and genera. The test is based on
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Length–weight relationships
Total length (L) and total weight (W) relationships were estimated for males, females and both sexes combined by fitting non-linear regressions to the data. The model fitted was W=aLb and the resulting parameters (a,b) were obtained using the Newton algorithm from the Microsoft ® Excel Solver routine.
Reproduction
Sex determination of small fish was difficult and hake smaller than 15 cm long were considered indeterminate. The sex ratio was therefore obtained from this length onwards. The spawning season was determined by macroscopic examination of the gonads for maturity stage according to a four-point scale validated histologically by Lucio et al. (2000).
Size at maturity was defined as the size at which 50% of males and females become mature (L50) and was estimated using specimens over 15 cm long collected during the months of maximum reproductive activity (December–May). The percentage mature by length class and sex were fitted to a logistic function using the Newton algorithm from ® Microsoft Excel solver routine
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Results |
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Age estimation
The pattern of annual otolith increments could be clearly seen by experienced readers along the dorsal–ventral axis of otolith sections. Interpretation of the otolith pattern of growth presented two main difficulties: the central area close to the nucleus, where the growth pattern of the first year of life is laid down, and the area beyond the sixth annual ring. As mentioned in the previous section, the central area possessed a number of "false rings". Taking into account the age interpretation methods described earlier, the first annual ring (1) was identified despite the presence of checks (–3, larval; –2, pelagic; –1, demersal) around the nucleus (Figure 2). Although the position of the first annual ring varied, its recognition was aided by the frequent presence of a well-marked translucent ring along the dorsal–ventral axis of the section, between two first annual bands.
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Growth increments between the first, second and third annual rings had two translucent checks per year (winter and summer rings). These false rings were not clearly discernible in subsequent years. In older fish, the increments were narrower and more visible on the ventral apex, with a progressive shift in growth towards the internal face. The area beyond the sixth annual ring was complex, the frequent appearance of faint lines between annual rings combined with the gradual reduction in the spacing between them made defining a consistent reading method difficult.
Due to the high incidence of checks, the edge of hake otoliths did not strictly follow the expected pattern of translucent bands in winter and opaque bands in summer. Translucent edges appeared all year round. On average, more than 60% of the whole otoliths had translucent edges, indicating a high incidence of checks particularly in summer samples (Figure 3). Overall, two peaks of translucent edges per year were observed in whole otoliths. The most important occurred in winter (November) and the secondary one in spring–summer (April–June). These two peaks appeared later in otolith sections, the first in February and the second in July. However, the optical characteristics of the thin sections under variable light could have affected these results.
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Of the total number of otoliths read, 12.4% were rejected for being unreadable. Precision estimates gave an average percent of error between readers of 7.7% and a mean coefficient of variation of 12%.
Growth parameters
The mean length at age for males, females and sexes combined are presented in Table 2. The maximum age recorded was 9 years for males and 11 years for females. Males larger than 60 cm total length were not caught and all fish older than 9 years were females. The sample was dominated by specimens belonging to age groups 1–5. Kruskal–Wallis tests comparing length distributions for each age of males and females showed highly significant differences from age 3 onwards (p<0.001). Comparison of mean lengths at age estimated by other authors with this study (Table 3) showed that in the study area, hake of any given age are larger than was estimated before, reaching mean sizes close to those estimated for hake of the Northern stock.
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The estimated growth parameters of males were L
= 70.0 cm; K=0.184 and t0=–0.973 and of females L = 88.7 cm; K=0.127 and t0=–1.157 (Table 4). For the sexes combined, the resulting parameters were L = 88.0 cm; K=0.128 and t0=–1.174. For the option where L was fitted to the observed data (120.5 cm), the results were K=0.075 and t0=–1.715. The estimated growth curves were plotted together with those estimated by other authors (Figure 4).
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The phi-prime test (
') was used to compare the growth parameters L and K estimated for the sexes combined with those obtained in other studies (Table 4). Within the same geographic area, ' values obtained from our growth parameters and those of Robles et al. (1975) were the same, while those of Iglesias and Dery (1981) and ICES (1991) gave lower ' values. For hake from the Bay of Biscay (Northern Stock) similar (ICES, 1993) and higher ' values were reported (Lucio et al., 2000). Finally, for waters off Morocco Goñi (1983) also calculated lower ' values.
Length–weight relationship
The total length–total weight relationships were: W=0.013L2.813 for males, W=0.008L2.942 for females (Figure 5), and W=0.00733L2.981 for the sexes combined. Females were heavier than males. Comparison of weights at age of males and females showed highly significant differences (p<0.001) from age 3 onwards. Estimates of parameters of the length–weight relationships for combined sexes are presented together with those of other authors in Table 5.
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Sex ratio
The sex ratio was close to 1:1 in hake smaller than 45 cm (Figure 6). Males outnumbered females in the size range of 25 to 45 cm, after which females predominated and rapidly increased in relative abundance to reach 100% in fish larger than 60 cm.
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Spawning season and size at maturity
Resting stage females appeared in April and disappeared in September, with mature females increasing from December to May, with a peak in February. Mature males were present almost throughout the year, although the highest proportion occurred from February to April, and a small amount of resting males were present from June to July and from September to October. Immature individuals were frequent throughout the year.
The estimated mean size at first maturity of males was 32.8 cm and the range of maturation (L75–L25) was 4.2 cm. The females' mean size at first maturity was 45.4 cm and the range of maturation (L75–L25) was 6.2 cm. The mean size at maturity for the sexes combined was 37.9 cm and the range of maturation (L75–L25) was 11.5 cm (Figure 7). The estimated age at which 50% of individuals become mature was 2.5 years for males, 4.4 years for females and 3.2 years for the sexes combined. The reproductive parameters of hake obtained by different authors and in different areas are summarised in Table 6.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionConclusionsReferences |
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Age interpretation
The wide range of length at age estimates of European hake obtained by different authors reveals the lack of agreement in the interpretation of ring patterns and the difficulties inherent to ageing European hake otoliths. The ageing criteria used here have been elaborated on the basis of research studies carried out by our team and have been universally accepted among hake otolith readers from European Institutions involved in hake ageing (Anon., 2000).
Every ageing method requires validation (Beamish and McFarlane, 1983) but due to the difficulties in accomplishing this in European hake, only age corroboration methods (Campana, 2001) have been used to support the age estimates presented here. Thus, Piñeiro and Hunt (1989) and Piñeiro and Pereiro (1993) provided evidence in support of the length range of the age one group by means of length frequency analysis and back-calculation of otolith ring measurements. Additionally, a recent study (I. Meneses, pers. comm.) on daily growth of juvenile hake from Portuguese waters (Southern stock) found that otoliths of fish 13–20 cm long caught between June and October presented three checks and 279–413 daily increments (Anon., 2000). Furthermore, in the Mediterranean Sea, studies of the otolith microstructure and length frequency analysis have found a mean size of 16 cm at the end of the first year of life of juvenile hake (Morales-Nin and Aldebert, 1997). The ageing criteria used here were also supported by the progression of the 1993 to 1999 year-classes in the Southern stock (Anon., 2000) from which the age estimations were obtained independently from two different laboratories (Figure 8).
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The characteristic checks or "false rings" observed in hake otoliths from different geographic areas are related to biological or seasonal environmental changes (spring–summer rings) affecting a large proportion of the population. Some of these checks have been identified in this study. In particular, the check between the first and second annual rings may be related to the feeding behaviour of this species. In the study area, Olaso (1994) and Velasco and Olaso (1998) described a drastic change in the diet of hake 15–20 cm long from 88% invertebrates (crustaceans) to 97% fish. This event could explain the presence of this check since it appears independently of the season when fish attain about 19 cm (Goñi and Piñeiro, 1988). In the past, this ring may have been interpreted as the second annual ring leading to underestimation of hake growth (Robles et al., 1975; Iglesias and Dery, 1981; Goñi, 1983; ICES, 1991).
The second check, observed between the second and third annual rings, is formed mainly in summer and at present there is no clear explanation of its significance. However, differences in the interpretation and counting of these translucent rings appear to be the main source of discrepancies in estimates of size at age. The shift in pattern of otolith growth from forming mainly two translucent rings per year during the first three years of life, to forming only one thereafter, has been associated with the process of sexual maturation (Hunt, 1980).
The seasonal pattern of translucent and opaque edges of hake otoliths found in our study, with a high incidence of translucent edges all year round, has also been reported by other authors (Iglesias and Dery 1981; Goñi, 1983; Piñeiro and Hunt, 1989). The edges of otolith sections were difficult to interpret, and we therefore recommend using whole otoliths when interpreting marginal growth. Nevertheless, further studies of fish larger than 40 cm are required to clarify this matter.
The complexity of the pattern of ring formation in otoliths indicates that European hake grow in an intermittent manner, undergoing a series of growth interruptions during the first years of life. In addition to the diet changes mentioned earlier, some of these interruptions could be related to the depth stratification of hake in relation to size and age (Pereiro and Fernández, 1983; Fariña and Abaunza, 1991). The predatory and reproductive behaviour of hake results in seasonal movements through habitats with different environmental conditions. The presence of false rings has also been reported in European hake from other Atlantic waters (Hickling, 1933; Descamps and Labastie, 1978; Goñi, 1983) and from the Mediterranean (Morales-Nin et al., 1998). A similar pattern of false rings has been observed in otoliths of other species of the genus Merluccius, such as M. bilinearis (Hunt, 1980) and M. hubbsi (Renzi and Pérez, 1992).
Growth pattern
The mean lengths-at-age obtained in this study indicate that hake from the Southern stock grow faster than it has been traditionally believed. For any given size, hake is now about 1 year younger than previously estimated. This would seem realistic and coincides with studies on food consumption using bioenergetics models that indicate that hake in the study area exhibit rapid growth rates (Riis-Vestergaard et al., 2000). This new length-at-age estimate makes the number of year classes that contribute to the catch smaller and the age at first maturity lower (ICES, 2000). It has important implications for the estimation of essential hake population parameters, such as recruitment, fishing mortality and spawning stock biomass.
Males grow slightly faster than females up to age 2 and from age 3 onwards female growth rates surpass those of males. Females also reach larger sizes and grow older than males. The difference in growth rate coincided with the onset of sexual maturity. Lucio et al. (2000) are the only authors who have described the growth and reproduction of hake simultaneously. They noted a similar pattern of differential growth between males and females, with males growing faster than females before reaching sexual maturity and the reverse afterwards. Greater and faster growth in females than in males is a common phenomenon in many species of demersal fishes (Landa and Piñeiro, 2000) and may be related to differences in metabolism between sexes, such as differences in oxygen consumption (Pauly, 1994) and/or to differences in the level of surplus energy between reproduction and somatic growth (Rijnsdorp and Ibelings, 1989).
Bearing in mind that comparison with other growth studies can be difficult due to differences in age estimation procedures, a visual comparison suggests that hake growth rates from this study are higher than previous estimates by Iglesias and Dery (1981) and ICES (1991) in the same study area, and much higher than growth rates obtained by Goñi (1983) in hake off Morocco. However, growth rates obtained here were lower than those estimated by Lucio et al. (2000) in the Bay of Biscay (Northern Stock), although similar rates have been reported for hake elsewhere in the Northern stock (ICES, 1993). We contend that these discrepancies are primarily due to differences in the otolith interpretation criteria used by the various authors, although factors such as sampling strategy used could also play a role.
The main features of the sex ratio observed in this study were a ratio close to 1:1 up to 45 cm and a predominance of females thereafter, reaching 100% after 60 cm. There is a moderate increase in the proportion of males in the interval of 25–45 cm TL which is due to the slowdown of the males' growth rate after the onset of maturity and the subsequent accumulation of individuals in that size range. A similar pattern of sex ratio by size has been described by Pérez and Pereiro (1985) in the study area, by Lucio et al. (2000) in the Bay of Biscay and Fariña and Fernández (1986) in western Ireland.
These results suggest that the natural mortality rate of old males may be much higher than that of females as males older than 9 years disappear from the fishery. However, the reason may be related to the different growth rates of males and females and/or to their different behaviour and consequently different accessibility to the fishing gear. Roff (1982) suggested that having reached the size at which the fish can successfully reproduce, the growth of males in some species of fish declines in part as a response to the divergence of energy into reproduction, and in part, as a response to decreased foraging activity. If male hake grow at a lower rate, particularly after the onset of maturity, the combined effect of growth and size-specific mortality would lead to a higher proportion of females in the larger size classes.
Length–weight relationships
In general, the estimates of length–weight relationships obtained here are close to those from previous studies. Differences in the numbers of samples at length distribution margins, especially in larger sizes, can explain the small differences with values reported in other studies.
Spawning season and size at maturity
The spawning season of hake in this study area extends from December to May and peaks in February. However, because hake is a partial spawner (Sarano, 1986; Murua et al., 1996) mature, immature and resting females occurred simultaneously during this period. These results agree with those of Martin (1991) and Lucio et al. (2000).
Estimates of the size at maturity of hake obtained in this study fall within the range of estimates obtained for hake from other areas. However, our results suggest that the size-at-maturity in the Iberian waters has decreased over the last years. Sustained high fishing mortalities in this stock may be responsible for this change (ICES, 1996; Goñi, 1998; Rochet, 1998).
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Conclusions |
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TopIntroductionMaterial and methodsResultsDiscussionConclusionsReferences |
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The successful application of ageing criteria presented here demonstrate that, with experience, otolith sections may be used to estimate hake age reliably up to age 5. Despite these advances, our ability to age European hake is still not good enough. Hake is a long-lived species reaching ages over 10 years and older fish are difficult to age. Thus, further research on alternative methods for ageing, and on age validation, are still required for hake. However, overall it is recognised that a significant improvement has been attained with the establishment of ageing criteria for the first 5 years of life. These criteria are already in use by the readers of the countries involved in hake stock assessment (ICES, 2000). This is particularly important because fish from ages 0 to 5 years make up 95% of the catch (mean of 1991–2000). Thus, this represents an essential step towards an accurate assessment of the status of the Southern stock.
Many efforts have been made to improve the knowledge of biological parameters of European hake in the Northeast Atlantic, but so far most of them have not produced sufficient validated information on which to base stock assessment. Our findings indicate that for European hake the age estimation of the first 5 years of life is reliable and that it will allow the successful application of empirical ALKs for the assessment of the Southern stock. Our estimates of size at maturity of hake from Iberian waters indicate there has been a decrease over the past few years, a worrying trend that will require further investigation as it may indicate over-fishing. Our results further indicate that hake from the Southern stock grow at higher rates than previously believed, approaching the values of the Northern stock. This result supports the hypothesis of Sanchez et al. (ICES, 1999) that there is no biological basis for European hake from ICES areas being separated by Cap Breton canyon into two different stocks (Anon., 1999).
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Acknowledgements |
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This work was funded in part by EU Study Contract BIOSDEF Ref. no. 95/038. The authors wish to thank the ship-owners, skippers and fishermen for their collaboration during sampling on board. We wish to thank all colleagues involved in the ICES Programme of IEO: Oceanographic Centre of Vigo, La Coruña and Santander, who collaborated in obtaining the samples and data. We are grateful to the colleagues from AZTI (Basque Country, Spain) and IPIMAR (Lisbon, Portugal) who have contributed with unpublished data. We also owe our thanks to C. Fariña, V. Trujillo and N. Pérez for their helpful comments on the draft of the manuscript and especially to R. Goñi for her valuable suggestions and for the revision of the text.
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References |
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