Seasonal variability of gillnet selectivity in Chilean hake Merluccius gayi gayi (Guichenot, 1848)

This study examines the seasonal variability in gillnet size selectivity for the main Chilean hake (Merluccius gayi gayi) fishery off the coast of central Chile. Selectivity estimates were based on five gillnet mesh sizes (50, 60, 70, 80, 90 mm) in four traditional fishing areas, and during periods of low and high reproductive activity. Average fish length caught in the earlier period (low reproductive activity) was 30.8 cm LT (±4.17), and significantly longer (p < .05) at 35.9 cm TL (±6.15) in the latter period (high reproductive activity). For males, the selectivity factor was similar in both periods, thus the estimated modal lengths did not differ seasonally (p > .05). For females, however, both the selectivity factor and the estimated modal lengths changed significantly depending on the time of year (p < .05). A significant increment was estimated in the girth/perimeter ratio and condition factor during the period of higher reproductive activity. This change in the fish girth of females during spawning helps explain the significant reduction in their selectivity factor estimates in August–October.     

Genetic differentiation in Chilean hake Merluccius gayi gayi (Pisces: Merlucciidae)

The genetic structure of Chilean hake Merluccius gayi gayi, was analyzed using starch gel protein electrophoresis. Samples were collected from four localities along the coast off Chile. A total of 1500 specimens sampled from Coquimbo, San Antonio, Talcahuano and Puerto Montt were used in the study. Genetic information was obtained for six allozyme loci Pgi-1, Pgi-2, Pgm, Idh-1, Idh-2 and Aat. The results of the analysis showed no significant allelic differences among samples from various localities, with an average F_ST value of 0.007 for all loci and samples. A heterogeneity test for all loci and specimens from the four localities showed only two significant values. Thus, Chilean hake populations along the coast of Chile appear to be genetically homogeneous.     

Size selectivity of three diamond mesh codends for the European hake (Merluccius merluccius) and the tub gurnard (Trigla lucerna) in the Sea of Marmara, Turkey

The effects of an increase in the codend mesh size from 44 to 56 and 80 mm on size selectivity of the two by‐catch species (the European hake Merluccius merlucius and the tub gurnard Trigla lucerna) commonly captured in the shrimp fishery in the Sea of Marmara, were evaluated. The parameters of the logistic selectivity model were estimated by maximum likelihood. Length at 50% retention, L₅₀, was found to increase with mesh size for both species. The L₅₀ values of 44, 56 and 80 mm were, respectively, 19.8, 21.5 and 33.4 cm for European hake, 17.9, 17.7 and 25.6 cm for tub gurnard. The main conclusions are that 44 mm and 56 mm diamond mesh codends retain a considerable amount of undersized hake and tub gurnard as by‐catch in the shrimp fishery. With 80 mm mesh size codend, most juvenile European hake are released. For the tub gurnard, however, even this mesh size does not provide a good enough selection. From the biological point of view, it is concluded that the legal 44 mm diamond mesh codend does not provide satisfactory selection for European hake and the tub‐gurnard in trawl fisheries of the Sea of Marmara.     

Length weight relationships of two small indigenous cyprinid fishes,Osteobrama cotio (Hamilton, 1822) and Salmophasia phulo (Hamilton, 1822) from Panchet Reservoir,Damodar River (tributary of River Ganga), India

Length–weight relationships were determined for two small indigenous cyprinid fishes: Osteobrama cotio (Hamilton, 1822) and Salmophasia phulo (Hamilton, 1822) collected from Panchet reservoir, Damodar River, India. About 143 specimens of Osteobrama cotio (5.2–10.4 cm TL) and 113 specimens of Salmophasia phulo (2.9–8.2 cm TL) were collected from November 2014 to June 2016 using gillnets of mesh size ranging from 25 to 120 mm (STR) and dragnets of mesh size 10–20 mm (STR). No previous information on LWRs for these two species from an Indian reservoir was available in the literature.     

Selectivity of diamond, square and hexagonal mesh codends for Atlantic horse mackerel Trachurus trachurus, European hake Merluccius merluccius, and greater forkbeard Phycis blennoides in the eastern Mediterranean

Knowledge on selectivity is considered essential for an adequate management of the trawl fishery. This paper compares the selectivity of conventionally used codends of 44 mm diamond mesh, 40 mm square, and hexagonal mesh for Atlantic horse mackerel, European hake and greater forkbeard in the eastern Mediterranean. Data were collected using the covered codend method and analyzed using the logistic equation with the maximum likelihood method. Mean selection curves were analyzed and compared using the between-haul variations model. The mean L ₅₀ v alues of diamond, square and hexagonal mesh codends are 14.7, 15.9 and 17.1 cm total length (TL) for horse mackerel, 10.4, 14.4 and 11.0 cm TL for hake and 12.2, 15.8 and 12.7 cm TL for greater forkbeard, respectively. For horse mackerel, the differences among the codends are statistically significant (P   <   0.05); however, the L ₅₀ values for all three meshes are higher than the minimum landing size (13 cm TL). For hake and greater forkbeard, the L ₅₀ value for the square mesh codend is significantly different from the values for diamond and hexagonal meshes (P   <   0.01), whereas there is no significant difference between diamond and hexagonal meshes (P   >   0.05). Regardless of the mesh shape, L ₅₀ values were substantially lower than the minimum landing size or size at first maturity for hake and greater forkbeard. To enable more effective solutions, research on species-selective trawls is required for the Mediterranean demersal trawl fishery.     

Trawl Selectivity Trials on the Deep-water Rose Shrimp (Parapenaeus longirostris) in Sicilian Waters

The selectivity of the traditional commercial bottom trawl net employed in Sicily to catch the Mediterranean deep-water rose shrimp, Parapenaeus longirostris, has been assessed. Two fishing campaigns were carried out in the Strait of Sicily and in the Southern Tyrrhenian Sea, using the covered cod-end method (mesh 20 mm vs. 31 mm). Of a total catch of 11,601 individuals, 23.4% escaped in the cover; the sample length structure from the Strait is unimodal, while that from the Tyrrhenian, polymodal. A logistic curve, fitted with a maximum likelihood criterion, has been used to model the selectivity data, in order to obtain the parameters CL_c50% (50% retention size), SR_75–25% (selection range) and SF (selection factor, i.e. CL_c50%/mesh). The two sets of data, besides a larger selection range for the Strait sample (5.2 mm vs. 2.3 mm), produced very similar estimates (retention sizes of 13.0 mm vs. 12.8 mm CL), fitting the logistic curve well. Almost no shrimp larger than 20 mm does escape from the cod-end; moreover, from the amount of damaged specimens found in the cover, even the evaded shrimps sustain a high fishing mortality. An increase of the present cod-end mesh opening, even above the size required by the EU bylaws (at present, 40 mm stretched) seems necessary for managing the fishery.     

Selectivity of a 50-mm diamond mesh knotless polyethylene codend for commercially important fish species in the Aegean Sea

This research investigated effects of changing the diamond mesh size on codend selectivity in Mediterranean fisheries. The selectivity of a typical 50‐mm diamond knotless polyethylene (PE) codend used in the Turkish fishery in the Aegean Sea was measured for commercially important species, in particular hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), anglerfish (Lophius piscatorius) and John Dory (Zeus faber). Fishing trials were carried out on the commercial trawler ‘Hapulo?lu’ between 9 and 12 December 2006 using a modified trawl net. Selectivity data were collected by the covered codend method and analysed by means of a logistic equation (Maximum Likelihood Method). The mean selectivity curve was estimated from individual hauls, taking between‐haul variations into account. Mean mesh size of the codend was 49.4 mm as measured by digital calliper. Mean values for 50% retention length of hake and horse mackerel were estimated to be 11.4 and 15.6 cm total length; corresponding selection ranges were 4.1 and 5.5 cm respectively. The 50‐mm diamond mesh codend showed adequate selectivity compared to the minimum landing size (MLS) for horse mackerel, while for hake it selected specimens in a size range far lower than the MLS. No selectivity values could be determined for anglerfish or John Dory. To design a more selective codend for the Turkish demersal trawl fishery, not only mesh size regulations but also other codend characteristics and netting material properties must be urgently considered.     

Length-weight relationship of coral reef-associated fishes from Gulf of Mannar and Palk Bay,Southeast Coast of India

Length-weight relationship parameter was estimated for eight coral reef-associated finfish species from the Gulf of Mannar and Palk Bay, Eastern Indian Ocean. Fish samples were collected from the landings of trawlers with a cod-end mesh size of 25–35 mm, operated at a depth ranging from 50–70 m off Tuticorin, Rameswaram and Thondi coasts on monthly basis between June-2018 and July-2019. Fish specimens were sampled by measuring total length (TL) and total weight (TW) with precision to 0.1 cm and 0.1 g respectively. A new maximum total length was recorded for Canthigaster solandri, Centriscus scutatus and Halichoeres stigmaticus from this study.     

Length–weight relationship and condition factor of Lepidocephalichthys goalparensis Pillai and Yazdani, 1976 in Assam,India

The present study describes the length–weight relationship (LWR), condition factor (K) and size at first maturity of a cobitid loach, Lepidocephalichthys goalparensis Pillai and Yazdani, 1976 of the family Cobitidae in Assam, India. A total of 716 specimens (males = 324, females = 392) ranging from 3.06 to 7.01 cm total length (TL) and 0.17–2.27 g body weight (BW) were analyzed. The overall value of the allometric coefficient b for the LWR indicated negative allometric growth (<3.00) for both males (2.774) and females (2.993). The size at first maturity (TL₅₀) for female L. goalparensis was estimated to be 5.6 cm TL. The condition factor values ranged from 0.56 to 1.35 in mature females. This is the first time that the biological parameters have been studied in this species.     

Efficiency of flexible sorting grids to improve size selectivity of the bottom trawl in the Balearic Islands (western Mediterranean), with comparison to a change in mesh cod-end geometry

In recent years several studies have been developed to improve the selectivity of the Mediterranean bottom trawl fisheries, which exert high fishing pressure on young individuals. The focus has been mainly on increasing the mesh size in the cod‐end or changing its mesh geometry; few studies have assessed the efficiency of the sorting grid systems. Analysis of 21 trawl hauls carried out at depths of 117–697 m off the Balearic Islands (western Mediterranean) in October–November 2005, studied size selectivity of the trawl using flexible sorting grids with 15 (SG15) and 20 (SG20) mm bar spacing. A divided polyamide bottom trawl designed for commercial purposes demonstrated high efficiency for the simultaneous use of both sorting grid sizes at different depth intervals (DS: deep shelf; US: upper slope; MS: middle slope). The results were also compared with those from the authors’ previous studies in the area where 40 mm diamond (DM) and square (SM) mesh cod‐ends were used. A saturation effect was detected on the DS for both sorting grids, due to the large amount of biomass captured at this depth interval. Size selectivity of 11 species was modelled, which showed an increase in length at first capture (L₅₀) from SG15 to SG20. Values of L₅₀ estimated for the main target species were: 10.9 cm total length (TL) with SG15, and 18.9 cm TL with SG20 for Merluccius merluccius; 21.1 mm carapace length (CL) with SG15, and 23.8 mm CL with SG20 for Nephrops norvegicus; and 15.9 mm CL with SG15, and 19.6 mm CL with SG20 for Aristeus antennatus. Comparison of size selectivity parameters between the sorting grids and cod‐ends of different geometry showed clear differences. For most species, the highest value of L₅₀ was obtained with SM (e.g. 26.6 mm CL for N. norvegicus and 22.1 mm CL for A. antennatus). Exceptions were the round‐fish M. merluccius, the flatfish Lepidorhombus boscii, and the crustacean Parapenaeus longirostris, with values of SG20 (18.9 cm TL, 12.1 cm TL, and 25.7 mm CL, respectively) higher than with SM (15.2 cm TL, 10.2 cm TL, and 20.2 mm CL, respectively). Sorting grid selection ranges were broader than those estimated for DM and SM. Experience shows that the exchange of the DM for the SM and/or the introduction of SG20 can improve selectivity in the Mediterranean trawl, SM being more efficient than SG20 in the Balearic Islands.     

Length and weight relationships of five different fish species from northern areas of Pakistan

In present study five different species were studied from three different rivers from northern Pakistan. Specimens were collected in the period of January to December 2017 using cast nets (10–12 mm mesh size) for Barilius, Glyptothorax and Schistura species while cast nets (mesh size 1.25 × 1.25 cm) used for Schizothorax species. Total length (TL) and fork length (FL) to the nearest 0.1 cm and body weight (BW) to the nearest 0.1 g were recorded for each individual.     

Oocytes development,maturity classification,maturity size and spawning season of the red mullet (Mullus barbatus barbatus Linnaeus, 1758)

The present study was designed to identify maturity of female red mullet, Mullus barbatus barbatus, using microscopic criteria, offering an accurate determination of the ovarian cycle and estimation of maturity size. Histological analysis of 499 females from 13 monthly samplings was applied (Thermaikos Gulf, N. Aegean Sea, Greece). Oocyte development was divided into five stages, with a mean oocyte size range from 56 to 363 μm in diameter. Ovarian maturity was defined by the maturation stage of the most advanced oocytes and divided into four phases. By using the monthly changes in the percentage frequency of ovarian phases, the ovarian cycle of M. barbatus was divided into three periods: a long period of early oogenesis (November to February), a short period of vitellogenesis (February to April) and a spawning period from April through June with peak activity in May. Both gonadosomatic (up to 7.5%) and hepatosomatic indices (up to 2%) can be used together to predict the spawning period. Red mullet should be classified as a multiple spawning species. Females reached 50% maturity (Lm₅₀) at 11.4 cm and 95% maturity (Lm₉₅) at 15.5 cm TL. The Lm₅₀ values for female red mullet populations across the Mediterranean Sea tended to show differences ranging from 11 to 14.4 cm FL or 11.4–14 cm TL. The current European Union fisheries management plan stipulates a minimum landing size (MLS) of 11 cm TL for Mullidae spp. fisheries in the Mediterranean. This MLS is at the lower part of the Lm₅₀ range, thus allowing an extensive removal of immature juveniles. A revision of this MLS value is recommended to help ensure that red mullet stocks remain within safe biological limits.     

Trophic interactions and community structure in the upwelling system off Central Chile (33-39°S)

Trophic interactions and community structure in the upwelling system off Central Chile (USCCh) (33-39°S) are analyzed using biological and ecological data concerning the main trophic groups and the Ecopath with Ecosim software version 5.0 (EwE). The model encompasses the fisheries, cetaceans, sea lion, marine birds, cephalopods, large-sized pelagic fish (sword fish), medium-sized pelagic fish (horse mackerel, hoki), small-sized pelagic fish (anchovy, common sardine), demersal fish (e.g. Chilean hake, black conger-eel), benthic invertebrates (red squat lobster, yellow squat lobster) and other groups such as zooplankton, phytoplankton and detritus. Input data was gathered from published and unpublished reports and our own estimates. Trophic interactions, system indicators and food web attributes are calculated using network analysis routines included in EwE. Results indicate that trophic groups are aligned around four trophic levels (TL) with phytoplankton and detritus at the TL=1, while large-sized pelagic fish and cetaceans are top predators (TL>4.0). The fishery is located at an intermediate to low trophic level (TL=2.97), removing about 15% of the calculated system primary production. The pelagic realm dominates the system, with medium-sized pelagic fish as the main fish component in biomass, while small-sized pelagic fish dominate total landings. Chilean hake is by far the main demersal fish component in both, biomass and yield. Predators consume the greater part of the production of the most important fishery resources, particularly juvenile stages of Chilean hake. Consequently, mortality by predation is an important component of total mortality. However, fishery also removes a large fraction of common sardine, anchovy, horse mackerel, and Chilean hake. The analysis of direct and indirect trophic impacts reveals that Chilean hake is a highly cannibalistic species. Chilean hake is also an important predator on anchovy, common sardine, benthic invertebrates, and demersal fish. The fisheries heavily impact on Chilean hake, common sardine, anchovy, and horse mackerel. Total system biomass (B=476 t km⁻² year⁻¹) and throughput (T=89454 t km⁻² year⁻¹) estimated in the USCCh model are in accordance with models of comparable systems. Considering system attributes derived from network analysis, the USCCh can be characterized as an immature system, with short trophic chains and low trophic transfer efficiency. Finally, we suggest that trophic interactions should be considered in stock assessment and management programs in USCCh. In addition, future research programs should be carried out in order to understand the ecosystem effects of fishing and trophic control in this highly productive food web.     

Selectivity of commercial compared to larger mesh and square mesh trawl codends for four fish species in the Aegean Sea

Differences in size selectivity of commercial (40 mm diamond mesh, 40D), larger mesh (48 mm diamond mesh, 48D), and square mesh codends (40 mm square mesh, 40S) for hake (Merluccius merluccius), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and four‐spot megrim (Lepidorhombus boscii) were investigated in the Aegean Sea. The study was conducted using the covered codend method. Data were analysed taking between‐haul variations into account. Results showed that changing from a 40D to 48D codend significantly improved mean L₅₀ values, with increases of about 22% for hake, 8% for greater forkbeard, 20% for blackbelly rosefish, and 11% for four‐spot megrim (P = 0.000). A change from diamond to square mesh configuration with the same 40 mm netting also significantly increased the mean L₅₀ values, with 45% for hake, 36% for greater forkbeard, and 25% for blackbelly rosefish (P = 0.000). For four‐spot megrim the mean L₅₀ value was about 10% lower, but this difference was statistically insignificant (P > 0.05). Despite the increases in L₅₀ values, this study concludes that the selectivity of the 48D and 40S codends is still not sufficient to release fish smaller than length at first maturity for these four species.     

Spawning,maturity, growth and movement of Platycephalus fuscus (Cuvier, 1829) (Platycephalidae): fishery management considerations

The dusky flathead (Platycephalus fuscus) is an important teleost harvested by recreational and commercial fishers throughout its endemic distribution along eastern Australia. This study indicates that the species has an extended spawning period throughout the austral summer, with females in spawning condition occurring in lower estuarine and coastal waters. Total length (L₅₀) and age (A₅₀) at which 50% (±1 SE) of the population was mature was 31.72 (±1.08) cm TL and 1.22 (±0.44) years for males and 56.75 (±0.60) cm TL and 4.55 (±0.13) years for females. The von Bertalanffy growth parameters differed significantly between sexes; females grew faster and attained a greater maximum TL and age than males. The largest female was 98.5 cm TL (7.5 kg), and the oldest 16 years, whereas the largest male was 61.5 cm TL (1.58 kg) and 11 years of age. A tag‐and‐release study identified the exchange of sub‐adult and mature‐sized individuals among estuaries. Determinations of length‐based management regulations for the species are compounded by the large gender‐based differences in growth and length‐at‐maturity. Current minimum legal lengths of 30–40 cm TL protect approximately 3–9% of the female spawning population. Alternative management options, including harvest slot sizes, need to be investigated and tested.     

Population structure in Chilean hake Merluccius gayi as revealed by mitochondrial DNA sequences

Genetic variation and divergence among samples of Chilean hake Merluccius gayi, from three localities off the coast of Chile and one locality off the coast of northern Peru, were assessed using sequences from the control region of mitochondrial DNA. Homogeneity tests revealed occurrence of at least three distinct genetic stocks of M. gayi within the region sampled. Factors potentially contributing to genetic divergence among M. gayi probably include hydrodynamics and behaviour.     

Length‐weight relationships of three species in northern China

Length‐weight relationships were estimated for three species (Triplophysa microphysa, Triplophysa zamegacephala and Gobio coriparoides) belonging to two families and two genera from northern China. Specimens were collected using trap nets (mesh size 1 cm) in May, June and December 2017, and March 2018. Total length (TL) for each individual was determined to the nearest 0.1 cm by digital slide caliper and each body weight (BW) to an accuracy of 0.1 g using digital balance. The b‐values for LWRs varied from 2.583 to 2.876 (r² > 0.979). This biometric data and their relationships are relevant to the management and conservation for local fishes and fisheries.     

Length–weight relationships of nine freshwater fishes from six rivers of the Western Ghats of India

Length–weight relationships (LWRs) were estimated for nine freshwater fish species collected three times using a scoop net (mesh size 0.3–0.5 cm), cast net (mesh size 1–1.5 cm) and gillnet (mesh size 2.5–4.5 cm) from six rivers of the Western Ghats of India during August 2017–October 2018. The b values for LWRs varied from 2.862 to 3.656 (R² > 0.916 and p < 0.0001 for all species).     

Length–weight and length–length relationships,gonadosomatic indices and size at first maturity of Eugerres mexicanus (Steindachner, 1863) (Percoidei: Gerreidae) from the Usumacinta River,Mexico

This study examines the length‐weight (L‐W) and length‐length (L‐L) relationships, gonadosomatic indices, and size at sexual maturity for individuals of the endemic Mexican mojarra, Eugerres mexicanus (Steindachner, 1863) from the Usumacinta River Basin, Tabasco, Mexico. From April 2008 to January 2010, 360 specimens were examined, including 151 (41.9%) male, 179 (49.7%) female and 30 unsexed (8.3%) individuals. Overall female:male ratio was 1.2 : 1. Gonadosomatic index (GSI) values indicate increased reproductive activity during February, with significant differences between males (GSI = 2.07) and females (GSI = 5.48). The mean size at first maturity (L₅₀) was 17.3 cm TL for males and 20.5 cm TL for females.     

Length-weight relationships of 11 fish species from the Min River Estuary and its adjacent waters,Fujian Province,China

Length-weight relationships (LWRs) were estimated for 11 fish species from the Min River Estuary and its adjacent waters, Fujian Province, China. All samples were collected using gillnets (mesh size 45 mm) and bottom trawls (mesh sizes 45 mm at the main webbing and 25 mm at the cod end) seasonally in 2017 and 2018. The parameters for the LWRs were estimated, along with the basic Statistical analyses for total length and weight ranges as well as the 95% confidence intervals for the intercept (a) and the slope (b) of the regression lines. Further, the coefficient of correlation indicated that the LWRs were highly significant (all r² > .95, p < .001). This study provides the new maximum total lengths (TL_max) for Coilia grayii (33.0 cm), Johnius taiwanensis (20.9 cm), Johnius trewavasae (22.0 cm) and Cynoglossus sibogae (22.0 cm).