In southeastern Brazil, slipper lobsters (Scyllarides deceptor and S. brasiliensis) are caught by fleets trawling for pink shrimp (Farfantepenaeus brasiliensis and F. paulensis) and pots-and-traps fishing for octopuses (Octopus vulgaris). Eight hundred fifty-six landings of shrimp trawlers and 28 of the octopus fleet were monitored in the Santos region from May 2006 to April 2007. Additional analysis was performed using a database covering the period from 1999 onwards. This study seeks to identify the recent patterns of exploitation of these lobsters with the goal of improving the way towards fishery sustainability. Scyllarides deceptor was the dominant lobster species with 1032 specimens collected, while only three specimens of S. brasiliensis were identified. The area known as the ‘Farol do Boi’ (23°01′S, 45°00′W to 25°00′S, 45°40′W at 60–135 m deep) showed the highest Catch Per Unit Effort (CPUE). A General Linearized Model (GLM) was used to investigate the factors influencing variations in CPUE in trawl fleets and led to the conclusion that year, month and depth were the most important factors. We detected a significant decrease in the relative abundance of lobsters in the fishing zone despite relatively low fishing effort. Recommendations to protect the lobster resources include taking special precautions in the natural refuge area of the ‘Farol do Boi’, as an exclusion zone for trawl fleets, and controlling the use of traps longlines to catch octopuses. Concerns about depensatory processes due to the over-exploitation of lobster populations around the world are raised.
Two sampling methods, equal probability and probability proportional to x (ppx), were investigated in order to recommend an optimum sampling strategy for carrying out a discards survey in the Irish Sea. The objective of the survey, which took place from April 1997 to August 1998, was to estimate the quantity of fish discarded at fleet level by Northern Ireland trawlers. Additional data collected from English and Spanish trawlers were used to test the generality of the results. Simple random, regression, ratio and ppx estimators were evaluated. The regression, ratio and ppx estimators use additional information on a variable x in order to improve the precision of the estimate of the variable of interest y and offer a potential improvement over the simple random estimator if there is a significant correlation between x and y. In order to compare the various estimators, 1000 independent data sets were generated using a bootstrapping technique for both equal probability and ppx with replacement selection. When comparing the estimators three criteria were examined: (i) bias, if any, of the estimator; (ii) relative precision of the estimators; (iii) accuracy of the formula for the variance of the estimator. Linear relationships between potential x–y combinations were assessed for significance and the residuals about the fitted line examined. This was carried out for each country and gear type for the main commercial species individually and all species combined. Different estimators proved optimal according to the x–y combination examined. In some instances the improvement in precision when using the ppx sampling method was marginal over the equal probability method. For the data sets used in this study the ppx scheme offered insufficient advantage over the simpler equal probability method to justify the greater complexity in implementing ppx.
Identification of periods of high and low cod production, and the reasons for these periods, can increase understanding of variability in populations and ecosystems. In this study we investigate the multi-decadal and multi-century scale variations in the cod population in the eastern Baltic Sea (ICES Subdivisions 25–32). Analytically derived estimates of biomass are available since 1966. These estimates show that biomass increased in the late 1970s–early 1980s, but decreased nearly 10-fold until the early 1990s and is still well below the long-term average. Prior to 1966 the biomass of cod is unknown, as is the relative role of fishing, climate variability/regimes, eutrophication and reduction of marine mammal predator populations. We have begun to investigate whether historical fisheries information (landings, effort, distribution) from before the 1880s is available in Baltic archives and museums, and to what extent this information can be used to interpret variations in this population. We have located fisheries data for different parts of the Baltic for different time periods since the 1550s and have interpreted the findings using current process knowledge of oceanographic mechanisms affecting cod reproduction and ecology in the Baltic Sea. The recovered data show that the Baltic ecosystem was able to support modest-large cod populations even though it was oligotrophic and contained large populations of cod predators (e.g., marine mammals). Current ecosystem management policy in the Baltic as developed and implemented by organisations such as the International Council for the Exploration of the Sea (ICES), the Baltic Marine Environment Protection Commission (HELCOM), the nine coastal countries and the European Union includes recovery of the cod population, a reduction in nutrient loading and measures to promote recovery of seal and harbour porpoise populations. If these policies are successful, the role of predatory fish in the future Baltic could again be substantial and comparable to that which we show existed 450 years ago. However, such a scenario will also require a major reduction in cod fishing mortality and suitable hydrographic conditions which promote successful cod reproduction. Historical ecology investigations in the Baltic can contribute to scientifically based fishery and ecosystem management and recovery plans.
This investigation reconstructs the development of major fisheries for herring, eel and whitefish in the Limfjord estuary, Denmark ca. 1667–1860, and then evaluates how their long-term dynamics have been influenced by some key fishery and environmental developments. The commercially most important fishery was for herring, Clupea harengus, which spawned in the Limfjord. This fishery underwent large changes in these centuries. High landings occurred in two periods in the early 18th and again in the early 19th centuries, when 4000–8000 metric tonnes were caught annually. In 1830, the fishery collapsed and landings were <1000 tonnes until the 1910s. Even during the 20th century using modern fishing techniques, the herring fishery never exceeded 7000 tonnes. The collapse was most likely due to unsustainable fishing practices (direct impacts on adults, juveniles, larvae and eggs). The second most important fishery of the Limfjord was the eel fishery. Eel, Anguilla anguilla, seems to have fled the fjord after a winter storm in 1825 broke the narrow Agger Tange isthmus which used to separate the Limfjord from the North Sea, and permanently increased the salinity in the western part of the Limfjord from 8 psu to 33 psu. The, so-called pulse seine fishery for eel declined rapidly following the salt water intrusion, and the population needed at least one to two generations to even partly recover. One possible technological reason for the recovery of the fishery was the 1848 invention of a new type of gear in the Limfjord which today is known as the Danish seine. However, the commercial eel fishery in Limfjord ceased by the 1980s coincident with the overall decline in European eel populations. The third fishery analyzed is the fishery for common whitefish, Coregonus lavaretus. A local population formed the basis of a substantial seasonal fishery, but the whitefish did not survive the salinity obstacle presented by the salt water intrusion in 1825. This study documents both the effects of fishing and environmental variability on collapses of different Limfjord fish species.
The reactions of vendace (Coregonus albula L.) shoals and single fish to an approaching pelagic pair-trawl deployed for mass removal purposes were studied by stationary hydroacoustic data acquisition and target tracking in the mesotrophic Bigge Reservoir (Germany) during November 2005. A Simrad EY 500 split-beam echosounder with a frequency of 120 kHz was used to observe fish in the mouth of the pelagic trawl. During six hauls with a duration of 20 min each, 61 single fish were detected and had a mean swimming speed between 0.7 and 2.4 m s−1 (mean = 1.5, S.D. = 0.4) which was comparable to the observed gear speed (1.1 m s−1). Calculated aspect-angles (fish tilt to the transducer axis) ranged between 74° and 102° (mean = 91.5, S.D. = 4.4) indicating that most of these fish showed horizontal swimming behaviour. The mean target strength (TS) of tracked fish varied between −54.2 and −32.9 dB and corresponded well to the length composition of vendace caught by trawling, as well as to a recently obtained species-specific TS–length relationship. Trawl catches comprised only vendace and varied between 10 kg (n = 350) and 60 kg (n = 2098). Whenever the trawl entered dense vendace shoals, single-echo detection and tracking analysis failed due to multiple-echo detections and single fish could not be distinguished. Based on the echograms the avoidance reactions of the vendace shoals were generally weak; the moment when trawlers stopped to haul the net was found to be the most critical point in the trawling process. Large numbers of fish were able to leave the gear at this stage and thus escaped capture.
Between 1992 and 2000, 681 blue sharks, Prionace glauca (173.8–310 cm total length, TL) were collected off northeastern Brazil (Brazilian EEZ). Vertebral sections of 156 males (173.8–310 cm TL) and 80 females (185.5–283 cm TL) were analysed. The index of average percentage error (IAPE) ranged from 0 to 3.1% for 3–12 growth rings (GRs). Mean marginal increment (MI) decreased from November to January without, however, conclusively confirm the annual pattern of deposition. Growth parameters were derived using the von Bertalanffy growth function (VBGF), the Richards function and the Schnute function. VBGF was that which best fit the data. Parameters derived from back-calculated lengths, K=0.157, L∞=352.1 cm and t0=−1.01 year, were considered to best describe growth. First maturity sizes of 225 cm for males and 228 cm for females corresponded to 5-GRs individuals. A 310-cm male had 12 GRs which was the highest number in the entire sample. Growth rates of 33.9 cm per GR were estimated from birth to the first band corresponding to 60.2% of size at birth, which is 56.4 cm. Adult males were 90.5% of the entire male sample and, adult females corresponded to 79.6% of the sample of females. The male sample was composed of individuals from 3 to >11 GRs and females were 3 to 10 GRs. Males were larger (with higher number of GRs) and outnumbered females in the overall sample, as commonly occurs in catches worldwide. Despite the hypothesis of differential growth by sex, postulated by several authors, the present study demonstrated that growth curves did not differ significantly, even considering significant vertebral radius (VR)–TL regressions for males and females. Higher growth rates were found in the southern Hemisphere, leading to significant differences in VBGF when compared to other areas. Discards and unreported catches are of great concern for conservation of this important apex predator.
Swimming activity of turbot was recorded in a 250 m2 earthen pond by acoustic tracking and was analysed in relation to illumination, temperature and oxygen concentration. A pronounced diurnal rhythm was found, with greater activity at night. Illumination interacted with both temperature and dissolved oxygen concentration in determining swimming activity. Activity increased with temperature, particularly above 20 °C at night, without a clear effect of oxygen concentration level ranging between 4 and 9 mg l−1.
The feeding habits of the striped red mullet, Mullus surmuletus on the Cretan shelf (north-eastern Mediterranean), in respect to season and fish size, were examined. Stomach contents of 446 specimens, 62–230 mm TL, collected by demersal trawl from August 1988 to August 1989, were analyzed. Feeding intensity was high throughout the study period and varied significantly among the size classes. Crustaceans (amphipods and decapods) predominated M. surmuletus diet. The composition of the prey ingested varied with predator size fish and cephalopods occurred exclusively in the diet of specimens larger than 161 mm TL. The mean weight of stomach contents increased significantly for fish larger than 171 mm TL, while the mean number of prey items did not differ among the size classes. Diets varied seasonally; decapods were more important in summer, whereas amphipods were more important during winter and spring. The results indicated that the red striped mullet fed on a narrow range of prey items and could be considered a specialist. The morphological characteristics and the foraging behaviour of M. surmuletus account for both prey type selection and the feeding patterns observed.
The Norway lobster, Nephrops norvegicus, is mainly targeted by bottom trawlers whose selectivity (mesh size = 40 mm stretch) with respect to this species is poor producing large quantities of discards both in terms of demersal fish species and undersized Nephrops. The Pomo pit (central Adriatic Sea) is a very important nursery area for European hake, Merluccius merluccius, but is also the main trawling ground for the Adriatic fleet exploiting Nephrops. The use of baited creels may constitute an alternative to bottom trawling. This paper summarises experimental data collected from the western Pomo pit in order to compare bottom trawl and baited creel catches with the aim of evaluating the ecological consequences of creeling. Because of high scavenger activity in the area, the effect of this on the creel fishery was also investigated.A large fraction of the trawl catch was composed of juvenile undersized individuals of commercial species, the same was not true for creels. Composition of Nephrops catches differed markedly between creels and the bottom trawl. Scavenger activity was very high. The results indicate that ecologically the use of baited creels appears to be a valid alternative to bottom trawling but, unfortunately, may not comprise an economically viable solution in the investigated area, which is 30 nautical miles offshore.
To determine the spatial and seasonal variation of the catch-per-unit-effort of the skipjack tuna as an index of abundance, the logbook records of the international purse-seine tuna fleet that operated in the Eastern Pacific Ocean during 1970–1995 were used. Data were provided by the Inter-American Tropical Tuna Commission (IATTC). The catch-per-standard-day fishing (CPSDF), by year, quarter, and type of indicator was estimated for the operational area. The interannual and seasonal variation were significant, with two long periods in the fishing yields (1970–1982 and 1983–1995). In the first period, the highest yield was 1971 and the lowest in 1982. In the second period, the highest yield was in 1995, and the lowest 1983. Seasonally, the highest yields were obtained in the first half of the year, and the lowest during the third quarter. As result of the CPSDF distribution, the most productive areas are south of 10°N and east of 135°W.
Age, growth parameters and food of Squalus acanthias L., spiny dogfish sampled from the SE Black Sea were studied. First and second dorsal spines were used for ageing. Spiny dogfish from age classes 1 to 38 were identified with an age distribution of 2–38 years for females and 1–27 for males. The length–weight relationship was W = 0.0000004 × L3.5 and W = 0.0000008 × L3.32 for females and males, respectively. The estimated von Bertalanffy growth parameters were; L∞ = 137.73 (±1.22) cm, K = 0.1263 (±0.0073) (year−1), t0 = 1.67 (±0.25) (year) (r2 = 0.93) for females, L∞ = 123.78 (±4.85) cm, K = 0.168 (±0.024) (year−1), t0 = 2.62 (year) (r2 = 0.96) for males and L∞ = 136.76 (±1.42) cm, K = 0.133 (±0.006) (year−1), t0 = 2.15 (±0.178) (year) (r2 = 0.95) for sexes combined. They feed mainly on fish (IRI% = 97.63) and crustaceans (IRI% = 2.23). Whiting was the main prey item in all seasons (IRI% = 87.81) except for winter where they feed mainly on anchovy (IRI% = 96.46). The stomach fullness and dietary component were affected by the gear used as well as seasons. It was concluded that the abundance and availability of the prey are main factors for the spiny dogfish feeding strategy in the Black Sea.
We examined the discrimination of picarel stocks in the Aegean Sea. The results of multivariate analysis on 14 morphometric and six meristic characteristics per individual showed no difference between the two sexes. Only morphometric characters showed enough between-group variability for the analysis. Analysis of these characters did not reveal clear discrimination of populations between all six areas examined. However, we can discriminate two major population groups. The first includes the populations of the North Aegean (Kavala, Alexandroupoli and Thessaloniki) and the second includes those of the South Aegean (North and South Crete and Cyclades). The phenetic tree of Mahalanobis distances showed that populations of picarel caught in neighbouring areas appear relevant.
The growth of juvenile and adult European hake from the Bay of Biscay and the Great Sole were studied analysing the sagittal otolith macrostructure and microstructure. The right and left sagittae otoliths were statistically symmetric in all studied dimensions. In the nucleus, 65 Daily Growing Increments (DGI) were identified. In the samples analysed a growth rate of 0.61 mm day−1 and a total length of 22.2 cm at the end of the first year were estimated. The comparative analysis of the results from several authors on hakes from similar areas showed that the growth rate obtained by tagging studies was almost half of that estimated by otolith microstructure analyses. These differences were probably due to the different size ranges of hake used in the studies. The increase and decrease of DGI width of three juveniles in Spring–Summer and in September coincided with the rise and fall of the water temperature in the Bay of Biscay and with a bloom of phytoplankton observed in March.
Variability and changes in the individual life-history parameters of fishes are frequently overlooked, and it is assumed that all individuals mature, spawn, grow, and die at the same rates over their lifespans. Here, the variability in the individual growth of the rudimentary hermaphrodite Diplodus annularis (Linnaeus, 1758) is described using a Bayesian approach. This approach enables the inference of individual growth curves, even in a species of a relatively short lifespan, and revealed a biphasic growth pattern for this species. Conventional von Bertalanffy growth failed to fit the individual back-calculated lengths-at-age data well. A generalization of this model is proposed for accommodating one change in the growth rate at some moment of the lifespan of this species. This novel five-parameter model (L∞, k0, k1, t0 and t1, i.e., size at infinite age, initial and final growth rate, age at size zero and age at the change of growth rate) represents the different allocation of energy to somatic growth or reproduction, prior to and post-sexual maturity. Moreover, between-sex growth differences are described; juvenile fish display similar growth rates in both sexes, but mature females have lower growth rates than males. The detailed description of the growth of the D. annularis shown here can provide adequate input for future implementation of population dynamics models that take into account individual variability (e.g., IBMs, individual-based models). These models could facilitate the management of a species targeted by recreational fishery.
Turbot Psetta maxima (Linné, 1758) were used as a model species to test if flatfish populations can be enhanced through regular release of reared fish. During 1991–1998, 10,000 I-group fish (11–16 cm) and during 1993–995 around 100,000 0-group fish (4–6 cm), were released each year in the Kattegat, off the northern coast of north Zealand, the most wind and wave exposed site in the inner Danish waters. The larger fish were tagged external, numbered, T-bar tags, whereas the smaller fish were marked with alizarin complexone. The size at release of the reared fish was similar to that of their wild counterparts, although the 0-group released fish were smaller than the average wild fish. The released fish showed a much narrower length distribution due to regular size sorting in the hatchery. The growth of released fish marked with alizarin was similar to or higher than that of their wild counterparts, whereas the externally marked fish grew slower, probably as a result of the external tag. The mortality of the released fish was very variable and relatively high, but was comparable to that of the wild turbot. The releases constituted around a third of the 0-group and almost half of the I-group wild counterparts. Most of the recaptures were close to the release area. Migration of the cultured 1-year-old turbot was less than 10 km and they remained close to the shore during the first 2 years after release. This was followed by a sharp increase in migration distance with an offshore direction, possibly related to spawning behaviour. No apparent difference in depth distribution between cultured and wild turbot was evident and both groups showed the same differences in depth distribution during night- and daytime. No evidence of displacement of the wild stock was found based on the findings of similar growth, similar size distribution in the later year classes and constant ratio of reared and wild fish in the catches. The results suggest that release of reared turbot may result in an increase in fishery recruitment.
Basic biological parameters of cuttlefish, Sepia officinalis, in the English Channel are described from samples of commercial and research vessel landings made between April 1994 and September 1995. There was a significant difference between the length–weight relationship of male and female cuttlefish. Growth of both sexes was rapid and seasonal during the last 12 months of life. Males grew faster than females, and reached larger overall lengths and weights. Most males reached maturity before the start of their second winter, although the testis continued to develop until spawning took place the following spring. Female maturation began later and was more prolonged such that it was completed towards the end of the second winter. Adults of both sexes spawned after the second winter between late March and July. Commercial landings data showed spawning cuttlefish initially arrived on inshore grounds in the western Channel, but slightly later and in greater numbers on the inshore grounds of the middle and eastern Channel. The weight and value of cuttlefish landings made by UK vessels in the Channel increased greatly between the mid 1980s and the mid 1990s such that cuttlefish are now a major part of the earnings for several fisheries. The most important fisheries were the offshore beam trawls, and inshore otter trawls and nets. The exploitation pattern of each fishery is described from quarterly samples of commercial landings taken between April 1994 and March 1996. Each exploitation pattern is shown to be a function of the region fished, the catching gear employed, and the growth and migrations of the cuttlefish population. Some implications for stock management are discussed.
The age, growth, reproduction, and feeding of common stingray (Dasyatis pastinaca) were studied using 256 specimens from the İskenderun Bay (eastern Mediterranean). Females made up 43% and males 57% of the individuals. The total length of females ranged from 20.5 to 88 cm (8–51 cm DW), and of males from 20 to 73 cm (7–34 cm DW). The total length–weight and disc width–weight relationships were W=0.00144×TL3.31 and W=0.02168×DW3.26, respectively. The age data, derived from vertebrae readings, were used to estimate the growth parameters of the von Bertalanffy equation: L∞=121.5 cm, K=0.089, t0=−1.615. The maximum age was 10 years. The parturition time of common stingray occurred in summer (from May to September). Males matured at 43 cm TL (22 cm DW) and females at 46 cm TL (24 cm DW). The stomachs contained mainly Crustacea.
The Common Fisheries Policy (CFP) regulates access to fisheries resources in an attempt to guarantee their conservation, management and exploitation. Direct control of fishing effort is the method that has historically been applied for the management of the Mediterranean fisheries. We used classification trees, a machine learning method, to analyse the effect that different closed seasons have on the yield of European hake (Merluccius merluccius (Linnaeus, 1758)) in “North Alboran Sea”, which corresponds to a geographic subarea of the General Fisheries Commission for the Mediterranean. Various algorithms (CHAID, C4.5, ECHAID, QUEST and J48) were used to generate the classification trees. The study was carried out between 2001 and 2008. During every year fishing was closed for 2 months, which were not necessarily always sequential. Moreover, the months included in the closed season varied between years. When the closed season was in April and May or in April and October, the repercussion on the yield was positive, when compared to other monthly combinations of the closure. The positive effect of the closed season in April and May is more evident than that of April and October. Contrarily, May and June or May and October closed seasons have negative consequences in fishing yields. Finally, a closed season corresponding to March and April showed an intermediate effect between the aforementioned combinations.
We studied the reproductive biology, growth, mortality, recruitment pattern and yield-per-recruit to the fishery of the mudskipper Periophthalmus barbarus (= P. papilio) in the intertidal swamps of the Imo River estuary in the south-eastern Nigeria. Monthly variation in gonadosomatic index reveals that the males spawn once in a year from February to May and the females from March to May. The size at which 50% of the specimens matured was 10.2 cm (total length) for females and 10.5 cm for males while the median size at spawning was 10.8 cm for females and 11.9 cm for males. We collected and analysed 12 consecutive months length–frequency data using FISAT software. Fitting the seasonalized von Bertalanffy growth function to these data gave the following results: total length, , C=0.9, WP=0.2 of the year. Using the seasonalized catch curve procedure, the estimated instantaneous total mortality coefficient . The instantaneous fishing mortality coefficient , and the instantaneous natural mortality coefficient . Our computed exploitation rate E=0.68 showed that fishing pressure on the stock is high. This was supported by the results of our relative yield-per-recruit analysis which showed the predicted maximum exploitation level Emax=0.51 which was lower than the current exploitation rate (E=0.68).
The red bass is a large tropical reef fish (Lutjanidae, tropical snappers) that is harvested to varying extents throughout a widespread Indo-Pacific distribution. The aims of this study were to investigate the accuracy and precision of age estimates from transverse sections of sagittal otoliths and to assess effects on these of the geographic area of collection and otolith preparation method. Two independent validation studies suggested an approximately annual formation of annuli in otoliths, predominantly for otoliths with 4–10 annuli but also for one otolith with 29 annuli. Otolith sections produced exceptionally high annulus counts: up to 56 annuli for samples from the Great Barrier Reef (GBR), Australia; and up to 55 annuli from the Seychelles, indicating a high longevity for this species. The precision of otolith readings from the GBR (index of average percent error, IAPE = 3.21 ± 0.26 S.E.) was within commonly accepted bounds for age estimation (IAPE up to 5%) but precision of readings from the Seychelles was significantly lower (IAPE = 9.18 ± 0.47 S.E.) and outside of this “acceptable” range. Age-based biological parameters for red bass from the Seychelles should thus be applied with greater caution than those for red bass from the GBR. If basic demographic properties are assumed relatively constant across the wide geographic range sampled, however, results from the GBR could be used as more reliable preliminary data for precautionary management strategies in the Seychelles and elsewhere.
The population dynamics and reproductive biology of Lutjanus fulviflamma were investigated using biological and size-at-age data derived from regular monthly samples of commercial catches in the southern Arabian Gulf. Transverse sections of sagittal otoliths showed well defined structural increments consisting of alternating translucent and opaque bands which were validated as annuli. The maximum absolute age estimates were 12.8 yrs (males) and 14.3 yrs (females). The size-at-age relationships were highly asymptotic in form with the majority of growth being achieved by the second year beyond which there was little increase in size with age. There were differential growth characteristics between sexes with females increasing in size with age at a faster rate and growing to a significantly greater mean size-at-age than males. Fish were observed in spawning condition between April and July, and the mean sizes and ages at first sexual maturity were 16.7 cm LF (1.6 yrs) for males and 18.7 cm LF (1.9 yrs) for females. The relatively short lifespan, rapid initial growth, early attainment of sexual maturity and high natural mortality rate, suggest that unlike most of the larger Lutjanids, L. fulviflamma would be resilient to exploitation. Furthermore, the study demonstrates the utility of structural increments in sagittal otoliths as a foundation for age based approaches to the assessment of fisheries resources in the southern Arabian Gulf.
The age and growth of the marbled spinefoot, Siganus rivulatus Forsskål, 1775, were determined based on specimens collected from Antalya Bay (eastern Mediterranean), during the period from October 1996 to August 1998. Total length–weight relationship was estimated as W=0.0064L3.221 for females, W=0.0079L3.135 for males. Growth parameters of the von Bertalanffy equation were computed for females and males as: and , respectively. Age was determined by posterior body scale readings. Maximum observed age was 8 years, suggesting that the species is long-lived.
Growth characteristics for two lethrinids, Lethrinus lentjan Lacépède 1802 and Lethrinus mahsena, Forsskål 1775 in the Red Sea off Jeddah were studied. With ages determined by scale readings, back-calculated lengths up to the seventh year of life were obtained for each species. Regressions of fish length on scale radius were also computed. Growth in weight was highest during the third and fourth years of life for L. lentjan and L. mahsena, respectively. Within each species, males and females gave virtually identical growth rates. Three exponential equations were derived for each species, representing the weight/length relationship separately for males, females and the sexes combined. Condition factor (K) values were also determined and variations of these values with fish size and month of capture were studied. The parameters of von Bertalanffy's growth equation were estimated for the two studies. Sex ratio at age suggests sex reversal from female to male during the fifth and seventh year of life for L. lentjan and L. mahsena, respectively.
The population biology and status of Acanthopagrus bifasciatus and Argyrops spinifer in the Southern Arabian Gulf were investigated to derive information required for their management. Transverse sections of sagittal otoliths were characterised by incremental structures consisting of alternating translucent and opaque bands. Edge analysis revealed an annual periodicity of formation with the opaque zone being deposited during the summer months. Parameter values of the von Bertalanffy growth function fit to size at age data (males and females combined) were: k = 0.234 year−1, L∞ = 32.5 cm (LF), to = −2.2 years (A. bifasciatus) and k = 0.224 year−1, L∞ = 52.4 cm (LF), to = −0.44 years (A. spinifer). Both species were fully exploited below the mean size at which females attained sexual maturity (26.4 cm LF for A. bifasciatus and 26.9 cm LF for A. spinifer). The overall sex ratios were significantly female biased although the bias was removed above the size at sexual maturation, potentially as a result of sex change. Spawning occurred from January to April for both species. The fishing mortality of A. bifasciatus accounted for 10.0% of the total mortality which was well within the estimated safe harvest level (F0.1). For A. spinifer, the exploitation rate (0.407 year−1) was comparable to that at the estimated safe harvest level (0.401 year−1). Nevertheless, the high juvenile retention rates (50.1% for A. bifasciatus and 66.9% for A. spinifer) and the discrepancies between the mean sizes at first capture and those which would maximise yield per recruit, suggest that the existing mesh size regulations in the trap fishery need to be reviewed. The results of the study highlight critical resource base issues and provide the direction for the future management of these species in the Southern Arabian Gulf.
Red bass, Lutjanus bohar, is a large tropical snapper (Lutjanidae) that is harvested to varying extents throughout a widespread Indo-Pacific distribution. The objective of this study was to estimate vital life history characteristics (age, growth, maturity) of red bass on the Great Barrier Reef, Australia, relevant to its management. The maximum estimated age of 55+ years is the oldest reported for any tropical snapper to date. The sampling of red bass from different depth ranges resulted in different age frequency distributions, suggesting that many older red bass reside at greater depths. The fit of the von Bertalanffy growth model described a trend of relatively slow growth: LF (fork length; mm) = 630 × (1 − e−0.10t+3.05), with no significant difference in fitted parameter estimates between males and females. Female red bass matured at a much larger size (L50 = 428 mm) and older age (t50 = 9.39 years) than males (L50 < 300 mm, t50 = 1.46 years) and were reproductively active over many months, from August to April. These results suggest that the red bass has a relatively K-selected life history strategy among the tropical snappers, and fish in general. This type of life history strategy predicts slow rates of turnover and a susceptibility of red bass populations to rapid over-exploitation.
To validate the periodic formation of growth rings in the genital plates of the Chilean edible sea urchin Loxechinus albus (Molina, 1782), an experimental sea urchin population grouped in five size classes (less than 15.0 mm, 20.0–29.9 mm, 50.0–59.9 mm, 70.0–79.9 mm and more than 100.0 mm) was confined to intertidal pools of the Marine Reserve of Mehuin. Size and weight were controlled monthly, from March 1991 to March 1992. At the same time, 50 sea urchins were collected each month from the non-manipulated population, involving the entire range of size distribution present in the study area. From these specimens, the genital plates were taken for ring reading, edge analysis and marginal increase analysis. The data were fitted to the individual growth model of Von Bertalanffy by means of a non-linear method (Marquardt's Algorithm). Asymptotic test diameter (TD∞) ranged between 141.2 and 131.7 mm, K (intercept on the TD)-axis) between 0.164 and 0.104 and t0 (time at age 0; settlement on botttom) between −0.23 and −0.77. In the case of the growth equation for weight, asymptotic weight (W∞) ranged between 1207.3 and 740.1 g, and the relationship between test diameter and wet weight was W = 0.00054(TD)2.94. The regression analysis used for estimating the growth parameters showed no statistically significant differences (95%) between the manipulated and the non-manipulated populations (rings at present and past ages). Moreover, all the growth parameters overlap at the 95% confidence interval obtained from the non-linear fit, showing no statistical differences. On the other hand, a significant relationship was found between the number of plates of a single column of the coronal system and age, which could be useful for the identification of this important demographic parameter in non-destructive field studies.
A study was carried out to assess the size and sex dependent catchability of Palinurus elephas in traps by comparing trap catches to trammelnet catches in the Marine Reserve of Columbretes Islands (western Mediterranean) during three different periods. The results indicate that catches of P. elephas taken with traps do not adequately represent the size structure of the population and that the sex ratios are biased towards females. Trap selectivity primarily affected the estimates of the males’ size structure. The effects on the females’ size structure were limited by their restricted size range. No significant seasonal changes in female catchability in traps relative to nets were noted, but catchability of mature males in traps appeared to be higher before reproduction. Additionally, the catchability of male P. elephas in traps and trammelnets was highly reduced during moulting. As a result of the above, changes in population structure observed in P. elephas trap assessment surveys in the reserve appear to be primarily due to seasonal variations in male size structure (operating through trap selectivity) and behaviourally induced changes of male catchability in traps. We discuss the implications of these findings for population assessment. From the point of view of the exploitation and conservation strategy, traps are preferred to trammelnets because they allow for the escape of a certain fraction of immature P. elephas and they protect the largest males.
Experiments were conducted under controlled laboratory conditions to evaluate catch performance of traditional rush and modern plastic spiny lobster (Palinurus elephas) traps. Our results showed that there was no significant difference in the catch efficiency of the two kinds of traps used suggesting that plastic traps could play an important role in re-launching the local artisanal fishery.
Metazoan gill parasites of 30 albacore Thunnus alalunga caught in the Balearic Sea (western Mediterranean) were examined for parasites with the aim to evaluate their possible use as biological tags. A total of 9 species of parasites were found: 1 capsalid monogenean, 6 didymozoid trematodes and 2 crustaceans. Most of the parasites collected were didymozoids (95.8% of all specimens) and Didymozoon longicolle was the dominant species. Albacore is a new host record for Capsala paucispinosa and Didymozoon pretiosus, while Didymosulcus aahi, Didymosulcus dimidiatus, Nematobothrium latum and Rocinela sp. are for the first time reported from the Mediterranean Sea. Significant differences were found grouping data by host size, with lower infection levels in the larger sized fish, whereas no differences were found between host sex. Most of the parasites showed a high site selection: D. aahi, D. dimidiatus and D. longicolle had significant differences of prevalence between internal and external margins of gill filaments, and almost all specimens of Pseudocycnus appendiculatus were attached to the gill filaments of the second and third holobranchs. The usefulness of parasites as biological tags is discussed; particularly, D. longicolle and D. pretiosus could be used to separate Mediterranean and northeast Atlantic stocks of albacore.
The aim of this study was to determine the nutritional condition of Sardina pilchardus larvae and the percentage of larvae that were starving at the time of capture. The survey was conducted, during the spawning season, in April and May of 1991 and 1992, off the northern coast of Spain. An accepted fluorimetric technique was used to determine the concentrations of both RNA and DNA and to calculate RNA/DNA ratio for each larva. RNA/DNA ratios were related to the zooplankton biomass (>53 μm). Low percentages of starving larvae (RNA/DNA ratio less than 1.3) were registered, ranging from 0% to 2.5%. Results based on the mean and variance of individual larval growth rates showed that predation pressure was not increasing from 1991 to 1992. Based on the presence of sardine larvae in good condition together with low predation, a high recruitment was expected. However, the 1993 recruitment, from 1992 spring spawning, was very poor. This was associated mainly with unfavourable advection from the nursery area. Moreover, even low levels of starvation, such as registered, operating over long time periods could have considerable consequences for larval mortality.
The feeding habits of the Atlantic bonito Sarda sarda (Bloch, 1793) caught in the southern Tyrrhenian sea were studied. The specimens were collected by means of a surface gillnet on a monthly basis between March and December 2003. The importance of the different prey items was assessed utilizing several feeding indexes, while possible size-related and temporal variations of the diet composition were tested by means of DISTLM forward and NMDS analysis. Intact prey items were measured in order to determine predator–prey relationships. The results of this study showed that S. sarda is a piscivorous predator focusing on clupeiforms and more in detail on three species such as the round sardinella Sardinella aurita, the sardine Sardina pilchardus and the anchovy Engraulis encrasicolus. Other prey were recorded occasionally. No temporal significant variations in the diet composition were found. On the other hand, a variation in the diet of the Atlantic bonito was observed according to its length. The smaller specimens fed primarily on E. encrasicolus. As they grew, S. pilchardus and S. aurita became important prey species, with the latter becoming by far the predominant prey for the largest Atlantic bonitos. Finally, a significant and positive correlation between predator and prey length was found.
Two methods are currently available for age estimation in octopus beaks. They have been applied to the same specimen from a sample of 30 individuals of Octopus vulgaris caught in central-eastern Atlantic waters. These techniques aim at revealing growth increments in the rostrum sagittal sections (RSS) and lateral wall surfaces (LWS) of octopus upper and lower beaks. Both methods were improved to reduce the time of sample preparation and to enhance the appearance of the increments. For each individual, two independent readings were done for upper and lower beak sections, as well as for the lateral wall surfaces. Vertical reflected light (epifluorescence) and image analysis system were shown to be useful in the observation and analysis of the sequence of increments. Precision of the ageing, increment counts obtained by both techniques, and increment widths were discussed. Using upper beak RSS led to more precise age estimates, whereas preparing LWS was quicker and simpler, and revealed a higher number of increments. Therefore, our study recommends counting growth increments in LWS of beaks to age adult common octopus.
Some aspects of the biology and fishery of Octopus vulgaris caught by trawlers in the Balearic Sea (Western Mediterranean) are studied. The analysis of the size–frequency distribution followed the growth of specimens from January (6–7 cm ML) to August (11–12 cm ML). The sex ratio was estimated for each season and it was not significantly different from 1 : 1 in any of them. The stomach contents revealed that the octopus fed predominantly on crustaceans and fishes. Another octopus species, Eledone moschata, is present in this fishery but its catches were clearly lower than those of O. vulgaris. The analysis of the importance of these two species in relation to the rest of the commercial catch showed that octopuses represent between 20–40% of the total catch for trawlers. The highest catch rates (kg/h) were obtained in spring and at the beginning of summer. Time-series analysis of monthly catches from January 1981 to August 1996 showed two main oscillations. The lower one, with a periodicity of 12 months, reflects the annual biological cycle of the species; on the other hand, the higher one has a periodicity of 92 months, the time series available being too short to confirm the significance of this period.
During the late 17th century (1675–1696), which represents part of the coldest period of the Little Ice Age (known also as the Late Maunder Minimum), fishing took place at more than 20 localities along the coast of the Gulf of Riga. Tax records of the Riga Treasury College indicate that herring (Clupea harengus membras), flounder (Platichthys flesus) and eelpout (Zoarces viviparus) were the major target species at this time. The mean annual catch estimated as landing to market of these three major species was 122 tonnes (range 44–230), with an average herring contribution of 73%. Fish catches fluctuated substantially both at temporal (seasonal, annual) and spatial (sub-regional) scales. We suggest that the herring fishery was affected by climatic conditions: during the period of very severe winters (i.e., 1685–1696) landings peaked in the warm summer months and were significantly lower compared to the earlier period of generally higher climate variability and less severe winters (1675–1683). Socio-economic drivers were presumably responsible for the dynamics of the flounder and eelpout fisheries.
The age of scarlet seaperch, Lutjanus malabaricus, from deep waters (>100 m depth) on the continental shelf of north-western Australia was estimated by examining transverse sections of their sagittal otoliths. Ages were assigned based on counts of alternating opaque and translucent zones (annuli). The consistency of the readings showed that the otolith increments can be used for age determination. Otolith weight was strongly correlated with fish age providing confirmation that the opaque and translucent zones used to estimate age in this study are formed on an annual basis. Growth was found to be moderately slow. The maximum observed age for a male was 31 years corresponding to 802 mm FL. There was significant differential growth between the sexes in observed length-at-age, with males growing larger than females. The von Bertalanffy growth parameters were: for males, and for females. The annual instantaneous rate of natural mortality (M) was estimated to be 0.112. The life history characteristics of L. malabaricus indicate that this species has a low-production potential and hence is vulnerable to overfishing. Harvest strategies should, therefore, be conservative and fishery managers need to consider harvest refugia in those fisheries where size at first capture cannot be manipulated by modifications to fishing gear, such as the demersal fish trawl and fish trap fisheries of Western Australia.
The early life history of the black anglerfish, Lophius budegassa was investigated by otolith (lapilli) increment analysis. Samples of demersal juvenile L. budegassa ranging from 54 to 196 mm total length were collected during bottom trawl surveys in the central Adriatic Sea. By counting increments presumed to be deposited daily in the lapillar otoliths, 88 specimens of L. budegassa were successfully aged. Age estimates of juveniles ranged between 79 and 204 days, indicating that probably the pelagic phase of this species is relatively short and settlement occurs at less than 3 months of life. The analysis of check marks in the core area of lapilli enabled us to determine the period of endogenous feeding, which would last between 15 and 24 days after hatching. Back-calculated hatching dates and, consequently, the spawning season of L. budegassa in the Adriatic Sea was spread over a long period, lasting at least from February to June. The length at age relationship gave an estimate of mean growth rate of approximately 0.8–1.02 mm/day, indicating a faster growth rate of 0+ juveniles L. budegassa than previously thought. The implications of these findings on age estimates discrepancies between previous ageing studies on L. budegassa carried out using different calcified structure (sagittae or illicia) are discussed.
This study concerns the biology and growth of the species Lophius budegassa from data obtained on annual spring cruises from 1994 to 2002. The black anglerfish had a very wide distribution, appearing in all surveyed zones from depths of 15 to 757 m but with greater abundance between 100 and 500 m. Sexual segregation by depth was observed, since the male proportion increased with depth. The length–weight relationship was negatively allometric, with growth curves being different for males and females. The parameters of the specialized von Bertalanffy growth model were estimated by two methods: length frequency analysis on a distribution composed of 1301 individuals, and age readings of the 170 first rays of the dorsal fin (illicia) sections collected between 1994 and 1998. Other growth models such as generalized von Bertalanffy, Richards, logistic, Gompertz and Schnute, were fitted. The results obtained are compared with previous studies.
In the present study, the macroparasite assemblage infecting Helicolenus dactylopterus (Delaroche, 1809) in three different areas off the Portuguese coast was evaluated in order to assess their use as biological tags in stock identification. Fish were obtained from commercial landings in Azores, Madeira and mainland Portugal (Peniche), and were examined for macroparasites infections according to standard procedures. Parasite assemblage composition and the prevalence and mean abundance of each macroparasite taxa within each area were calculated. Amongst the 20 taxa found, 13 were selected as good biological tags, presenting statistically different infection levels between areas. Anisakidae larvae, which presented different prevalence and mean abundance levels between the three areas, were further identified according to the restriction fragment length polymorphism (RFLP) pattern on TaqI, HinfI and HhaI. The multivariate discriminant analysis applied to the macroparasites species present in more than one area and showing prevalence higher than 10% revealed a high differentiation among the three sampled areas suggesting at least three different bluemouth stocks in Portuguese waters.
The genetic and morphologic variation of Solea lascaris (Risso, 1810) was studied along the Portuguese coast through protein electrophoresis and morphometric and meristic analysis. Genetic analysis was based on 10 loci, all except one polymorphic. The average heterosigosity (HT) was 0.116, a high value but within the range found for several flatfishes, and the analysis of F-statistics showed that this variation is mostly due to differences within populations. Genetic distances between samples were low and seem to agree to an isolation by distance model. Morphologic analysis was based on 11 morphometric characters and 7 meristic counts. Results show a high overlap between samples. Both genetic and morphological analyses indicated a great variability but suggested low levels of differentiation along the Portuguese coast. Nonetheless, genetic analysis showed that these levels were significant and population structure should be analysed using markers able to detect a greater degree of population differentiation.
A study was made of the reproductive biology of Liza aurata (Pisces Mugilidae) from a local population in the lagoon of Klisova (Messolonghi, W. Greece), which revealed fundamental information about reproduction dynamics of this species. Gonadosomatic Index (GSI%) values were highest in September for both sexes and maintained this level from August–November. It was inferred that the spawning period extends through these months taking place in the sea, as all riping fish leave the lagoon in a seaward migration. First maturation occurred at age 1+ for both sexes. Fish of age 1+ comprised the greatest portion of spawners following by ages 2+ and 3+. Absolute fecundity varied from a minimum of 80 000 for age 1+ to a maximum of 1 410 000 eggs for age 7+. The absolute fecundity relations to fish total weight and total length were best described by the following exponential equations: F = 531 528 Ln(TW) − 3E + 06 and F = 2E + 06 Ln(TL) − 6E + 06, respectively. Fecundity–age relation was best described by the polynomial equation: F = 18 824A2 + 22 139A + 176 467.
Age based assessment forms the basis of management advice for the heavily exploited combined stock of blue whiting (Micromesistius poutassou) in the Northeast Atlantic. However, little historic attention has been given to the reliability of age estimates gathered from several nations involved in this assessment. Using recognised graphical and statistical approaches, bias and precision was investigated for experienced age-readers of blue whiting. Significant linear bias was found to exist between age-readers, with ages differing, on average, by 1 year for important year classes. Indications are that spawning checks and split rings affect the interpretation of annuli for some age-readers. An experience gradient became evident during the analysis; more experienced age-readers had greater levels of precision. Within reader precision was found to be higher than between reader precision; however, within reader bias was also evident with two out of three age-readers systematically revising ages downwards when re-ageing otoliths. Results indicate that differences exist on a limited international level for the age determination of blue whiting. This issue now needs to be addressed by the scientific community.
Information is presented on the occurrence of spinal deformities in natural populations of sandsmelt, Atherina boyeri in the Neretva River estuary, middle eastern Adriatic. During 1998 and 1999, the spinal deformity levels varied between 2.02% and 10.30% in four samplings, and 3.58% in total catch. The spine is deformed in one to a few places from vertical to horizontal angles, and the deformities are visible on the fish body immediately after catching. The possible reasons for such deformities are discussed.
Non-equilibrium production models were used to provide guidance/information on the exploitation level of the main deep-water crustacean fishery in the western Mediterranean. Annual catch and effort (engine horsepower) data from 1957 to 2001 were used to compute a long data series of catch per unit effort (CPUE) as kg/HP. Furthermore, a sampling program on-board the trawl fleet in the Balearic Islands was carried out between 1991 and 2001 to estimate the annual CPUE in kg/h. Both CPUE data series were used to fit non-equilibrium production models. Process-error estimators were applied to the Fox model and observation-error estimators to the Schaefer model. Parameter estimates obtained from the short CPUE data series were unreliable and highly uncertain for both models, mainly due to insufficient contrast in the data between 1991 and 2001. The parameter triplet k, r, and q (i.e. the carrying capacity, intrinsic rate of biomass increase and catchability coefficient), as estimated with the CPUE (kg/HP) data series from 1957 to 2001, gave 1814 t, 0.327 per year and 0.054 for the Fox model and 3656 t, 0.226 per year and 0.019 for the Schaefer model. These results were used to calculate biological reference points. Maximum sustainable yield, YMSY, was estimated at 218 t (Fox) and 210 t (Schaefer). Parameter estimates were less variable for the Schaefer model and these were therefore used for management recommendations. Currently, catches are 30% below the YMSY, suggesting that red shrimp (RS) is slightly under-exploited. A precautionary fishing effort was estimated taking into account the uncertainty of the parameters. Consequently, a fishing effort corresponding to a biomass level of 60% of the carrying capacity, f60%k, was adopted. At the current fishing effort, catches in 2002 remain at recent levels. Nevertheless, taking into account the precautionary approach in the fisheries, it is recommended that catches in 2003 should not exceed approximately 100 t, so that the long-term catch would correspond to f60%k, which is estimated to be around 200 t.Despite the simplifying assumptions of production models, the correct use of these models can provide an insight into species/fishery dynamics and can be used to analyse the effect of changes in the level of fishing level on catches. It should be noted, however, that the perception of the exploitation level of RS presented in this study should be further validated. It is recommended that the sampling program to collect data on fishing effort (fishing hours) should be continued allowing the non-equilibrium production models, both with frequencist and Bayesian approaches, to be applied to a longer CPUE (kg/h) data series.
Using daily catch data on the rose prawn (Aristeus antennatus) an attempt was made to construct a model relating catch per unit effort (CPUE) and a luminosity factor proportional to luminosity levels at the bottom on the prawn fishing grounds. A close relationship between this luminosity factor (defined in the text) and catchability was observed. Daily catch rates were highest during the 2 h immediately after sunrise, associated with a certain optimum light threshold. Within this pattern catch rates varied with longitude according to season and trawling depth. Based on these findings, novel alternatives for the future management of this resource are presented.
The present study updates the information on the red shrimp fishery in the waters of the Balearic Islands in the middle of the Western Mediterranean, from its beginning to the present. Also, the development of the fishery and the biology of the shrimp population exploited from 1992 to 1997 is analysed.The red shrimp (Aristeus antennatus) is one of the most important resources of bottom trawling in the Balearic Islands. It is fished on the slope between depths of 400 to 800 m. In biomass, it represents an average of 5% of the overall catches, but its economic value is 30% of the total earnings of the fishery. From 1948 to 1997, the landing increased at an average rate of 3405 kg per year. However, this increment was not constant for the whole period, and has been decreasing in the last five years. The engine power of boats fishing on the slope of the Balearic Islands doubled from the 1970s until the end of the study. At the moment, the number of fishing units is decreasing, although the power of the individual boats is increasing continuously. The trawl yields varied between 3 and 14 kg/h. The highest yields occurred in winter and spring. The decline of the sizes at first capture in 1997 and the increase of juveniles in the catches can be interpreted as a change in the fishery strategy, which has increased the exploitation effort on the small sizes.
The value of parasites and commensals of the northern scallops Argopecten purpuratus as biological tags for stock discrimination is assessed. Six-hundred and eighty-six specimens obtained from four wild populations in northern Chile (El Fraile, La Rinconada, Caldera and Tongoy) were analyzed. Eleven species (four parasites and seven commensals) were obtained. A canonical multivariate analysis suggests that scallops from northern Chile comprise four stocks. Identification of stocks is largely based on significant differences in the mean intensity of infection of Rhinebothrium sp. (Cestoda) and Dodecaceria sp. and Polydora sp. (Polychaeta)
Studies were conducted on the dynamics of the exploited population of Ethmalosa fimbriata (bonga) from purse seine fishery in the Cross River Estuary and adjacent Gulf of Guinea. Growth parameters were determined using the length–frequency distribution over an 18-month period. The asymptotic length, L∞=35.95 cm total length, von Bertalanffy growth function, K=0.90, winter point, WP=0.1, signifies that growth slows down in February which was attributed to high temperature and reproductive activities. The longevity was 40 months. The instantaneous rate of total mortality, Z=6.91 per year, natural mortality, M=1.61 per year and fishing mortality, F=5.30 per year, were estimated. The current rate of exploitation (E) was given as 0.77, indicating that the bonga stock is over-fished. From the yield-per-recruit analysis, Emax, the exploitation rate giving maximum relative yield-per-recruit was estimated as 0.65, confirming that the stock is being over-fished. To reverse the overexploitation of the bonga stock in the Cross River Estuary, and the attendant adverse socio-economic consequences, it was suggested that effort should be reduced in the fishery and the economy of the fishing communities diversified.
The distribution of and fisheries for bonga, Ethmalosa fimbriata (Clupeidae), in Nigerian inshore waters east of the Niger delta are discussed. Data on length frequency collected between 1973 and 1982 are analysed to obtain the von Bertalanffy's growth parameters (L∞=30 cm, K=0.43 and T0=−0.20) which are then used to estimate the instantaneous total (Z), natural (M) and fishing (F) mortality coefficients. Catch-per-effort statistics (1973–1982) allow for a simple graphical treatment of Schaefer's logistic model from which the maximum sustainable yield (Ymax=30 075 t year−1) is derived for the stock of bonga in this part of the Nigerian coast. Considering the level of mean catch (25 030 t year−1), which is very close to the optimum level of yield (Yec) and the mean value of the exploitation ratio (), it is concluded that the stock of E. fimbriata of the inshore waters east of the Niger delta is being fished at about its Yec. Although this is the desirable level of exploitation if profit is to be maximized, this fishery might be allowed to expand beyond Yec to contribute to the solution of an important socio-economic problem, namely, reduction of unemployment, but fishing at Ymax should be avoided to prevent the collapse of the stock.
During 13 trawl surveys carried out in the Sicilian Channel between 1985 and 1991, about 2300 specimens of Squalus blainvillei were caught. Ages were determined by examination of vertebrae, and growth parameters were estimated by a statistical fit to the Von Bertanlaffy function. The estimated growth parameters are L∞ = 117.9 cm, k = 0.102 and t0 = −1.380 for females, and L∞ = 96.0 cm, fk = 0.135 and t0 = −1.397 for males. The length-weight relationship (W = aLb) and maturity stages were determined separately for each sex: for females a = 0.0037 and b = 3.0688, for males a = 0.0033 and b = 3.0919. The analysis of sexual maturity revealed that reproduction takes place throughout the year.
The population structure of Aphanius fasciatus in the Mesolongi and Etolikon lagoonal system was studied, using 5794 fish. Significant differences were observed in the number of individuals of each sex, the age and size composition and survival. The overall males to females sex ratio was 1:2.44, although there was seasonal variation. During the reproductive period the percentage of males in the population decreased significantly, while after reproduction and during recruitment they increased. The females in each age class were larger than the males (40.33 mm and 36.72 mean total length for females and males, respectively). The survival rate of females was greater than males (0.73 for females and 0.60 for males). The strategy of this species is to invest in female individuals.