Minimum Legal Size Research Articles

Reproductive cycle of the barred sorubim Pseudoplatystoma fasciatum (Linnaeus, 1766), in the upper delta of the Orinoco River, Delta Amacuro State, Venezuela

Pseudoplatystoma fasciatum represents one of the most important items in the continental fisheries of Venezuela. The great fishing pressure exerted on this species has decimated the populations in the Orinoco River Delta and other regions of the country. The current condition of the population in the delta region is unknown and information about basic biological parameters is unavailable. Hence, population and reproductive parameters of this catfish were determined in this region. Results showed that the size interval was from 30 to 105 cm total length (TL), generally smaller than the size range found in other regions of the country. The largest value for the gonadal index for females was 116, recorded in July. Size at maturity of 50% of females was 46.38 cm TL, a smaller value than others reported for the Orinoco River Basin. Maturation of oocytes was synchronous, showing a single total spawning per year. The capture of P. fasciatum was continuous throughout the year, including mature individuals during the reproductive season, suggesting a possible utilization of the delta region as reproductive site. A total of 61% of the individuals were below the minimum legal size, indicating deficient management of this fishery resource in the Orinoco River Delta.

Spatial and temporal variation in size at maturity for female American lobster in Nova Scotia

Using the development stage of cement glands to predict spawning, maturity ogives were constructed for six fishing ports in eastern Nova Scotia. The size at which 50% of females attained maturity (L50) and the ogive slopes were compared between locations and years. Among ports within years, 21 of 25 L50 pairs were significantly different, while only 5 of 24 slope pairs were different. These results fill a geographic gap and indicate that most of the areas should not be grouped when calculating egg production. Comparing among years within ports, most significant differences were again for L50 rather than slopes. In two ports, the change in the ogive among years gave a 2- or 2.4-fold change in eggs per recruit and a 2.9- or 5-fold change in the percentage of females that reach maturity by the legal minimum size. On a larger scale, L50 was intermediate in Newfoundland, consistently low throughout the southern Gulf of St. Lawrence, abruptly increased on the outer coast of Nova Scotia into the Bay of Fundy, and again intermediate inshore and offshore New England.

Recruitment hotspots boost the effectiveness of no-take marine reserves

No-take marine reserves are widely advocated as a means to conserve biodiversity and sustain fisheries. Within adequately protected reserves, the abundance, mean size and age of targeted fish and invertebrate species are often increased. However, the magnitude and rate of recovery within reserves varies among locations and species, and increases in abundance in reserves are not universal. One potential contributing factor to variability in reserve performance is spatial variability in the supply of recruits. Many reef fish species exhibit disproportionate levels of recruitment in relatively small areas (so-called “recruitment hotspots”). Here we tested the hypothesis that the presence of recruitment hotspots enhances the effectiveness of reserves for two important fishery species, coral trout (Plectropomus maculatus) and stripey snapper (Lutjanus carponotatus), on coral reefs of the Keppel Island group, Great Barrier Reef. To do this we compared fish densities in areas that did or did not contain previously identified recruitment hotspots, both inside and outside reserves. For both species, the mean densities of adult fish above the minimum legal size were approximately three times as high in reserves with hotspots than in reserves without hotspots. Furthermore, the mean densities in reserves without hotspots were similar to those in nearby areas that were open to fishing. In contrast, the densities of sub-legal size individuals of both species were primarily explained by the presence of recruitment hotspots, irrespective of reserve protection. Our results suggest that identifying and incorporating recruitment hotspots into the selection of sites for reserves could enhance both conservation and fisheries objectives.

Spatial and Size Distribution of Red Drum Caught and Released in Tampa Bay, Florida, and Factors Associated with Post-Release Hooking Mortality

The recreational fishery for red drum (Sciaenops ocellatus) in Florida is unusual in that most red drum targeted are immature and caught within estuarine waters. Current state regulations rely exclusively on bag and size limits, resulting in the release of a large proportion of captured individuals. This study employed hook-and-line sampling conducted monthly in Tampa Bay, Florida and catch-and-release mortality experiments to determine the spatial and size distribution of red drum and the mortality rate of released fish, respectively. Of the 1,405 red drum collected, more than 70% were smaller than the minimum legal size (457 mm standard length (SL)). Size structure of red drum varied spatially and reflected ontogenetic patterns of habitat use. Data collected during catch-and-release mortality experiments were analyzed to identify factors associated with mortality. A total of 251 red drum (203-618 mm SL) were caught and held for 48 h during 9 experiments, with an overall mortality rate of 5.6%. Higher water temperature and anatomical hook position were significantly correlated with mortality; lip-hooked fish had the lowest mortality rate, while throat-hooked fish had the highest. Although hook type was not correlated with mortality, it did influence whether a fish was deep-hooked. Fish caught by J-hooks were more likely to be deep-hooked than those caught by circle hooks. Catch-and-release fishing is an effective management tool for reducing take but may contribute to short-term mortality, especially in warm, subtropical estuaries.

Impact of lobster size on catches and prediction of commercial spiny lobster landings in Brazil

On the Brazilian continental shelf, red spiny lobsters, Panulirus argus (Latreille, 1804) constitute a separate population with two distinct stocks, one in shallow waters (<50 m) and one in deeper waters (50-100 m). Stocks are highly exploited, especially undersized animals (≤75 mm CL) and older individuals. The observed decline in spiny lobster landings over the past two decades is believed to be due to overfishing in shallow waters, since over half the harvested lobsters are below the minimum legal size (75 mm CL), with a length at first capture of 61 mm CL. Spawning lobsters in deeper waters measure 96 mm CL at first capture; this coincides with the size class (90-99 mm CL) with the greatest reproductive potential. Furthermore, the capture of berried females and older lobsters (>135 mm CL) from deeper waters increases the risk of low recruitment and fishing collapse. The observed change in fishing patterns appears to be the consequence of increased fishing mortality (F) among pre-recruits. This is possibly the main cause of the large fluctuations observed in landings, although other factors, such as the interplay between environmental conditions and recruitment abundance, may have to be considered. Our results show that increasing the minimum legal size of red lobsters is likely to improve total landings in the long term, provided appropriate and tightly enforced control measures are designed and implemented.

Deep-water red coral from the island of Sardinia (north-western Mediterranean): a local example of sustainable management

Since 1979, red coral harvesting in Sardinia has been regulated by regional laws. The regulations, including a minimum take size of 10-mm basal diameter, were imposed to limit and control harvesting. Our study indicated that >75% of the red coral colonies harvested in 2007–08 in the north-western coast were above the legal minimum size for exploitation. Moreover, extensive surveys with remotely operated vehicles allowed us to document the real structure of wild populations. Large colonies (>10 mm) represented a big portion of the population (~38% of the total), suggesting that harvesting effort did not affect yet the maximum size of colonies. All these results seem to be confirmed by the application of the model of Beverton and Holt. This model shows that the fishery of red coral in Sardinia reaches its maximum yield at an age at first capture of 20 years, an age significantly lower than the average age of 30 years calculated for the colonies actually harvested in the Sardinian coast. Our data confirmed that the measures enacted in the Sardinian regulations provide for an adequate management of this resource and, in addition, can represent a guideline for other countries that still lack a valid management plan.

Reproductive biology of the blue jack mackerel,Trachurus picturatus(Bowdich, 1825), off the Canary Islands

Summary The reproductive cycle of the blue jack mackerel, Trachurus picturatus, had not been described for the Canary Islands. Between March 2005 and March 2006 monthly samples of T. picturatus were collected randomly at the central fishery wharf from the commercial catches of purse-seiners in Tenerife Island waters (Canary Islands). Some 2472 specimens were analysed, with total lengths from 10.4 to 31.9 cm. Although females outnumbered males in summer, males were more abundant in the sex ratio (1.36 : 1). Based on the monthly evolution of the gonado-somatic index and the proportion of mature individuals, the spawning season occurred between January and April, peaking in February. Lengths at first maturity (LFM) were calculated from the maturity ogives by the Gompertz model for all specimens (22.79 cm), for males (21.20 cm) and for females (23.05 cm). In this area the minimum legal size for T. picturatus is actually smaller than the length at first maturity and should be revised to avoid depletion of the stock.

Estimating a new suitable catch size for two clam species: Implications for shellfishery management

The present study analyzes the weight gain patterns per unit of size and estimates the minimum suitable catch size of two clam species: the carpet shell clam Ruditapes decussatus and the Manila clam Ruditapes philippinarum. For this purpose, data from the two largest estuaries along the northern coast of Spain (Cantabrian Sea) were used. The length–weight relationship of both studied species was estimated using two models: a classic allometric model and a nonparametric regression model based on local linear kernel smoothers. Additionally, first derivatives were used to estimate a minimum capture size for this species, corresponding to the size at which the first derivative reached the maximum. Within this context, the models application showed (a) the nonparametric model resulted in a better fit of data for both species (b) different minimum catch sizes for each species based on maximum length (49.5 mm for R. decussatus and 44.7 mm for R. philippinarum), both considerably larger than the currently established in EU and (c) an effect of estuaries and zones on individuals weight gain patterns. This confirmed the nonparametric model as an alternative approach to analyze the length–weight relationship for the studied species and to estimate a minimum suitable catch size of capture. The application of the specific catch sizes obtained in this study could lead to an increase in available commercial stocks of both species and positive effects on the conservation of the native species. In addition, the estimated zone-specific weight-gain patterns and minimum catch sizes could support a zone-based management. Therefore, this study provides a preliminary study and a starting point to consider the revision of the minimum legal size of the studied species in order to improve the current shellfishery management model.

Differential distribution pattern of native Ruditapes decussatus and introduced Ruditapes phillippinarum clam populations in the Bay of Santander (Gulf of Biscay): Considerations for fisheries management

The aim of the present study is to provide a first characterization of the grooved carpet shell clam Ruditapes decussatus (native) and the Manila clam Ruditapes phillippinarum (nonindigenous) populations in the Bay of Santander in order to improve the management of these commercially exploited resources. For this purpose a field survey was carried out in different fishing areas where samples were taken on transects, following artisanal shellfisher exploitation techniques. Biometric relationships, size frequency distributions, densities and stocks were evaluated for different fishing zones. In addition, a hydrodynamic model was applied in order to understand larval transport and recruitment patterns associated to the tidal currents and water flow. Within this context, the first evaluation of the clam populations in the Bay of Santander showed: (a) that fishing activity is performed on individuals under the minimum legal size (40 mm) and in closed areas, (b) a significant differences on density by zone (c) a distribution pattern with areas where both species coexist and areas where one of them dominates, (d) R. decussatus occurs at relatively low density in stations near the culture parks and (e) a limited recruitment in the inner parts of Cubas tidal fresh for R. phillippinarum and in the southern zones for R. decussatus. Based on this study, some managing guidelines are presented mainly focused on avoiding the overfishing of the native clam R. decussatus.

Ovarian follicular development in the hawksbill turtle (Cheloniidae: Eretmochelys imbricata L.)

Ovarian follicular development is an essential process in the determination of maturation stages associated with size. This association acquires importance when managing populations of threatened species. We histologically processed 11 prepubescent ovaries, four pubescent ovaries, and one breeding adult ovary with vitellogenic follicles using specific staining techniques to identify the follicular stages of Eretmochelys imbricata. Follicular stages were compared with maturation stages [including straight carapace length (SCL)]. The ovary presented several germinal beds and a lacunar system less histochemically and morphologically heterogeneous than that of crocodiles. During previtellogenesis (four stages), the oocyte grows rapidly due to the strong transcriptional activity of lampbrush chromosomes and numerous nucleoli, and the strong metabolism associated with lipid synthesis. The Stage III ooplasm showed a Sudan positive band. This stage was the most frequent in all ovarian sections and it was independent of maximal follicular stage. Stage IV, more frequent in pubescent and adult ovaries, presented a lipid vacuole-rich ooplasm and a broadening of the zona pellucida and the theca. The vitellogenesis begins with the penetration and accumulation of spherical glycoprotein yolk platelets and chemically neutral lipid droplets which are observed to be mixed, but spatially and chemically segregated. Both the yolk platelets and lipid droplets increase in size, density, and proximity to the periphery of the oocyte due to their coalescence. The SCL of the immature females did not determine the maximal follicular stage nor its frequency in the ovaries. Straight carapace length turned out to be an imprecise measure in identifying the presence of follicular stages in females larger than the minimum legal size limit in Cuba. Consequently, for a national conservation program to be successful, it must emphasize the critically endangered status of E. imbricata, as well as the maturity of the most frequent hawksbill sizes encountered in the Cuban archipelago fishery.

Comparing selectivity of a standard and turned mesh T90 codend during towing and haul-back

In this study, we compared the size selectivity of a T 90 codend (netting turned by 90 degrees) with that of a standard codend made of similar netting. Sea trials were conducted in a Norway lobster directed fishery in the Kattegat-Skagerrak area, where there is a need for improved selectivity because of a severe discard problem. The codends were tested by fishing simultaneously with them in a twin trawl rig. Codend covers mounted with Minisamplers were used, which made it possible to catch individuals escaping during towing and haul-back separately. Herein we proposed a model to assess the sequential selection during towing and haul back. This model takes into account the parameter C tow , which can be interpreted as the proportion of fish that comes into contact with the codend meshes during towing and, thereby, has a chance of escape. Compared to the standard codend, the T 90 codend retained fewer Norway lobster both below and above the legal minimum landing size (40 mm, cephalothorax length), thereby causing a reduction of commercial catch. The difference was mainly due to a significantly higher escape rate during towing for the T 90 codend. For plaice below minimum landing size (27 cm), the retention was slightly but significantly higher for the T 90 codend compared to the standard codend. A model developed for both codends showed that not all plaice are able to attempt escapement during the towing process. For cod, the results indicated an increased L 50 (the length at which 50% of this species is caught) for the T 90 codend, but the effect was not statistically significant, probably due to the limited number of cod retained during the sea trials. The results demonstrated that, for all three species, a significant proportion did escape during haul-back in both codends.

Assessing barotrauma among angled snapper (Pagrus auratus) and the utility of release methods

Australian recreational and commercial fishers catch snapper (Pagrus auratus) from down to ∼120m, often evoking barotrauma. Owing to minimum legal sizes and quotas or non-consumptive fishing practices, some afflicted fish are released, raising concerns over the potential for unaccounted fishing mortality. Two experiments were completed to quantify the (1) clinical signs of barotrauma among fish angled from 6 to 120m (experiment 1) and (2) mortality (over 3d) of fish with barotrauma that were released following either no treatment, venting (e.g. needle) or recompression (e.g. release weight) (experiment 2). In experiment 1, barotrauma was evident in some fish angled from 11m and all from >20m. Fish were considered to have barotrauma if they had a prolapsed cloaca combined with a distended coelomic cavity and/or gastric herniation into the buccal cavity. Despite similar clinical signs among fish in experiment 2, none died, however, the associated trauma raises welfare concerns. Both recompression and venting offered benefits over no treatment by returning fish to depth quickly and releasing gases, respectively. Nevertheless, the release method should be dictated by the clinical signs of barotrauma and extrinsic conditions, including the competency of the fisher (e.g. for venting), presence of predators (e.g. for recompression), and exposure to sun or warm water (e.g. no treatment). More selective gears, spatial and temporal closures, or possibly replacing minimum legal sizes with quotas in deep water only could also reduce the frequency of barotrauma among released P. auratus and therefore, negative impacts on stocks.

Diversity of the edible fishes of the Lagos Lagoon, Nigeria and the public health concerns based on their Lead (Pb) content

The study was conducted in Stratum VII (Yeji sector) of Lake Volta where commercial fishermen arbitrarily use gillnets, from June, 2009 to March, 2010. The catches of monofilament gillnets, the most dominant gear used in Stratum VII were investigated for dominant fish species composition and selectivity parameters. The study revealed the dominance of two fish species (Synodontis membranaceus and Schilbe mystus) which together contributed about 45% by number of gillnet catches. The optimum selection length (total length) of the 5 cm mesh size gillnet for S. membranaceus was 19.7 cm and S. mystuswas 18.2 cm. The result showed that using monofilament gillnets with mesh sizes 3 and 5 cm could have increase pressure on juvenile population. The mean monthly catch for the study period was estimated to be in the range of 0.3961 × 10-3 – 3.2427 × 10-3 (standard deviation, δ=1003.9) metric tonnes. With reference to precautionary approach to fisheries management, a legal minimum mesh size of gillnets of 8 cm may be appropriate for conservation and sustainable exploitation in Stratum VII. Key words: Catch, gillnet, management, selectivity, species.

A software tool for monitoring legal minimum length of landings: Case study of a fishery in southern Spain

Abstract The regulation of minimum legal size (MLS) of catches is a tool widely applied in the management of fisheries resources, although the MLS does not always coincide with the length at first maturity (LFM). The optimization of this management tool requires a series of quality control in fish markets and transportation. A software application has been developed to make the control of the landings of several target species easier and faster. In order to test and make this tool operational, six species of commercial interest were selected: four species of fish and two species of bivalves. It is proposed to estimate the proportion of illegal specimens in the studied lot from the proportion of illegal individuals found in the samples taken from this lot. The input data for the application are the minimum legal size (MLS) of the species and the total length (TL) of each specimen sampled. The output data is a statistical summary of the percentage of specimens of size less than the legal minimum (TL≤MLS) within different confidence intervals (90%, 95% and 99%). The software developed will serve as a fast, efficient and easy to manage tool that allows inspectors to determine the degree of compliance on MLS control and to make a decision supported by statistical proof on fishing goods.

Parámetros reproductivos y poblacionales de Thais chocolata (Duclos, 1832) (Gastropoda, Thaididae), en la reserva marina La Rinconada, Antofagasta, Chile

Reproductive and population parameters of Thais chocolata that would contribute to the validation of the current extraction standards were obtained by performing a study in the protected area of La Rinconada Marine Reserve, Antofagasta, Chile, from December 2008 to January 2010. In terms of reproduction, the results revealed asynchronic gonad development in the population, with specimens in different stages of maturity throughout the year. Mature individuals were distributed between 5 and 13 m depth, forming aggregations at 5 m depth during much of the year. Periods of greater maturity were observed from July to August and November to January, with the most important aggregations at the end of both periods. A relationship was observed between the months with the greatest maturity and aggregations and the months with the highest daily temperature fluctuations. At the population level, the results allowed us to estimate 2.3*10 6 individuals, with 39% of this population over the minimum legal size (55 mm). The relatively slow growth estimated for this population was probably influenced by the high variability of the bottom temperature in this area. The critical size and size at first maturity of the population were higher than the minimal legal size, making it is necessary to review the current fishing regulation for this species. The effectiveness of the State marine reserves in the conservation of this marine resource was demonstrated.

Complexity of applying minimum legal sizes (MLS) of retention in an indigenous coral reef fishery

Abstract Retaining coral reef fish for subsistence during commercial fishing is a common practice for indigenous fishers in the Torres Strait, Australia, despite being inconsistent with legislation. Fisher access point surveys were completed between 2004 and 2006 on three islands in Torres Strait to characterise this subsistence practice and assess the level to which it undermines current minimum legal fish sizes. Approximately 15% of the annual total catch was retained for subsistence during commercial fishing. Notably, subsistence catch of the most commercially valuable species almost entirely comprised individuals smaller than the minimum legal sizes. The higher proportions of undersized individuals of valuable species retained during commercial fishing on some of the islands were most likely associated with an increase in professionalism of the fishers. These results demonstrate how the intended outcomes from a management strategy can be undermined when the specific operational conditions of the fishery are not considered. Successful implementation of management arrangements in indigenous communities ultimately depends on the sociocultural conditions of the communities and their understanding and adherence to the rules. A productive way forward for the management of this fishery is greater engagement of indigenous communities and managers in co‐management arrangements.

Nova Scotia's bloodworm harvest: Assessment, regulation, and governance

The Nova Scotia commercial blood worm harvest was first regulated in 2002; by 2008 regulations included limited entry, seasons, area closures, minimum legal size, and reporting of daily sales and fishing location. To aid in setting minimum legal size, size of maturity was measured in the two main harvest areas. Biologist-directed and harvester-directed methods for surveying worm densities and sizes on intertidal mud flats were found to be robust. With 16 samples tests of two survey means would detect differences of 0.5 standard deviations (type I error of 0.1). With 16 samples two means separated by 1.3 standard deviations would not be declared the same when they were actually different (type II error of 0.25). Depth of digging affected survey results; the time of air exposure of the mud, the temperature of the mud surface, and the choice of harvester-samplers did not. Mud flats should be managed individually because interchange of worms between flats is probably small. Decision rules of 0.6 and 0.8 legal sized worms/m 2 have been set for closing a beach and opening a closed beach, respectively. Progress in fishery governance was achieved using the above results and by involving harvesters and a citizens group in decision making.

Population structure and reproductive behavior of Sinaloa cichlid Cichlasoma beani (Jordan, 1889) in a tropical reservoir

The population structure and reproductive condition of the Sinaloa cichlid Cichlasoma beani from samples obtained from June 2000 to July 2001 were determined. Samples in the first week each month from the largest trader of tilapia in the Aguamilpa Reservoir in Mexico and were caught in gillnets (9.6 and 11.4 cm stretch-mesh size). Of 596 specimens, there were 427 males and 169 females; monthly sex ratio, frequency of lengths by the multinomial distribution, timing of reproduction, condition index, and size at first maturity was determined. Differences in the sex ratio and monthly totals were significant, favoring males, except for September 2000 and March 2001. From one (August 2000) to three modal groups (July 2000 and June 2001) were identified by size. There were no significant differences in standard length weight relationships by sex, which indicated that a shared model for both genders is appropriate, and isometric growth was detected. Based on the proportion of mature and partially matures fish, the main reproductive period was April through June; size at first maturity was 18.9 cm. Water temperature was not significantly related to the percentage of mature and partially matures Sinaloa cichlids or spawning. These findings provide information for regulating the Cichlasoma beani fishery in this region such minimum legal size and non-fishing period.

Declines in the abundance of coral trout ( Plectropomus leopardus) in areas closed to fishing at the Houtman Abrolhos Islands, Western Australia

This study aimed to assess temporal trends in the abundance and size of the tropical coral trout, Plectropomus leopardus (Lacepède, 1802), in areas open and closed to fishing at the Houtman Abrolhos Islands. Baited remote underwater stereo-video systems (stereo BRUVs) were used to conduct surveys from 2005 to 2009 from shallow and deep sites across four geographically-separated groups of islands. A total of 1542 P. leopardus were observed from 77% of stereo BRUV deployments with an average abundance of 2.35 ± 0.1 SE per deployment. The lengths of 1238 individuals were measured, with sizes ranging from 118.2 to 727 mm. At commencement of the study in 2005, there were 65% more P. leopardus in areas open vs. closed to fishing. From 2005 to 2009, however, abundances fell by 50% in areas closed to fishing, such that abundances were then similar between areas open and closed to fishing. Relative abundance changed very little in fished locations throughout the course of the study, despite the implementation of fisheries restrictions on other iconic species that may have increased fishing effort on P. leopardus. In areas closed to fishing 12–32% of P. leopardus measured over the five years of the study (average of 25%) were larger than the minimum legal size for retention of 450 mm total length. Much fewer were larger than legal size in areas open to fishing (6–15%, average of 11%). Illegal fishing is considered to be the primary cause of the decline in abundance of P. leopardus inside closed area boundaries.

Invertebrate Dive Fisheries Can Be Sustainable: Forty Years of Production from a Greenlip Abalone Fishery off Southern Australia

The South Australian greenlip abalone (Haliotis laevigata) fishery was established in 1964, and commercial catches have been stable at approximately 350 t.yr−1 for over 20 years. This contrasts the status of fisheries for abalone outside Australasia, where rapid over-exploitation and stock collapse, without recovery, have been widespread. This study of the sustainability of the South Australian greenlip abalone fishery is based on a range of fishery-dependent and fishery-independent data, and it contrasts with a previous assessment based on fewer data that suggested these stocks were overfished. This fishery's success demonstrates that prolonged harvests can be obtained from wild abalone stocks. Key elements of the management system critical in preventing over-exploitation include early limitations on access to the resource, establishment of appropriate minimum legal sizes, and effective controls on total catches. A biological research program that focused explicitly on stock assessment and underpinned timely adjustments to management arrangements based on changes in stock status has also been critical. Recommendations by fisheries management committees involving resource managers, scientists, industry members, and other stakeholders have been underpinned by formal management plans. Sustainability of benthic invertebrate fisheries vulnerable to overfishing could be enhanced by adopting elements of the management system underpinning the South Australian abalone fishery since 1968.