Population biology and assessment of the white-spotted spinefoot, Siganus canaliculatus (Park, 1797), in the southern Arabian Gulf

The age, growth, mortality, reproduction and resource status of Siganus canaliculatus in the southern Arabian Gulf were investigated using a combination of size frequency, biological and size‐at‐age data. Defined structural increments consisting of alternating translucent and opaque bands in transverse sections of sagittal otoliths were validated as annuli. The maximum absolute age estimate was 7.8 years. Parameter values of the von Bertalanffy growth function fit to size‐at‐age data (males and females combined) were: k = 1.0, L_∞ = 24.8 cm (L_F), t_o = −0.1 years. Fish in spawning condition were only observed between April and July although patterns in gonadosomatic indices suggested a second but less well defined spawning event in November. The mean sizes and ages at first sexual maturity were 21.5 cm L_F (1.9 years) for males and 25.7 cm L_F (2.1 years) for females. Fish were fully recruited to the fishery at a size (L₁₀₀ = 19.7 cm L_F) that was smaller than the sizes at which sexual maturity was attained. The annual instantaneous rate of fishing mortality (F = 0.85 year⁻¹) (0.26–1.44 year⁻¹ 95% CI) was considerably greater than the target (F_opt = 0.33 year⁻¹) and limit (F_limit = 0.44 year⁻¹) biological reference points, indicating that the stock is overexploited.     

Comparative genetic stock structure in three species of commercially exploited Indo-Malay Carangidae (Teleosteii,Perciformes)

We examine genetic structuring in three commercially important species of the teleost family Carangidae from Malaysian waters: yellowtail scad Atule mate, bigeye scad Selar crumenophthalmus and yellowstripe scad Selaroides leptolepis, from the Indo-Malay Archipelago. In view of their distribution across contrasting habitats, we tested the hypothesis that pelagic species display less genetic divergence compared with demersal species, due to their potential to undertake long-distance migrations in oceanic waters. To evaluate population genetic structure, we sequenced two mitochondrial (mt)DNA [650 bp of cytochrome oxidase I (coI), 450 bp of control region (CR)] and one nuclear gene (910 bp of rag1) in each species. One hundred and eighty samples from four geographical regions within the Indo-Malay Archipelago including a population of yellowtail from Kuwait were examined. Findings revealed that the extent of genetic structuring among populations in the semi-pelagic and pelagic, yellowtail and bigeye were lower than demersal yellowstripe, consistent with the hypothesis that pelagic species display less genetic divergence compared with demersal species. The yellowtail phylogeny identified three distinct clades with bootstrap values of 86%–99% in mtDNA and 63%–67% in rag1. However, in bigeye, three clades were also observed from mtDNA data while only one clade was identified in rag1 dataset. In yellowstripe, the mtDNA tree was split into three closely related clades and two clades in rag1 tree with bootstraps value of 73%–99% and 56% respectively. However, no geographic structure appears in both mtDNA and rag1 datasets. Hierarchical molecular variance analysis (AMOVA), pair wise F_ST comparisons and the nearest-neighbour statistic (S_nn) showed significant genetic differences among Kuwait and Indo-Malay yellowtail. Within the Indo-Malay Archipelago itself, two distinct mitochondrial lineages were detected in yellowtail suggesting potential cryptic species. Findings suggests varying degrees of genetic structuring, key information relevant to management of exploited stocks, though more rapidly evolving genetic markers should be used in future to better delimit the nature and dynamics of putative stock boundaries.     

Growth and maturation of the redlip parrotfish Scarus rubroviolaceus

This study presents age‐based life‐history information for the red lip parrotfish Scarus rubroviolaceus based on a 5 year sampling programme from the commercial fishery of American Samoa. Females reached sexual maturity at 31·9 cm fork length (L_F) and 2·6 years and sex change occurred at 42·3 cm L_F, although not all females change sex through their ontogeny. The maximum observed age was 14 years and c. 65% of the fishery harvest was above the median L_F at sex change.     

Comparison of a rigid grid to a square‐mesh panel in size selection for commercial fishing in the Persian Gulf shrimp trawl fishery

Two bycatch reduction devices (BRDs): a juvenile and trash fish excluder device (JTED) and a square‐mesh panel (SMP), were tested in the Persian Gulf shrimp fishery from a small‐scale shrimp trawler in October–November 2012. Each device was alternatively tested by using a small‐mesh cover net to retain individuals that escaped the BRDs. Three valuable fish species—tigertooth croaker (Otolithes ruber), silver pomfret (Pampus argenteus) and silver sillago (Sillago sihama)—were selected for detailed analyses. Average escapement rates by number for commercial species in the trawl nets with either JTED or SMP were calculated as 24% and 33%, respectively. There were significant differences in escapement rates for Otolithes ruber and Sillago sihama between JTED and SMP (p < .05). In both BRDs, Pampus argenteus had the smallest escapement rate. A two‐sample Kolmogorov–Smirnov test detected a significant difference (p < .05) in the length‐frequency of Sillago sihama, and Otolithes ruber captured in the codend and cover net equipped with JTED. Selectivity curves of both BRDs indicated that for each species, the length of 50% retention probability (L50) was substantially smaller than the length at maturity (Lm), except for Otolithes ruber. From an ecological perspective, the codend equipped with SMP was superior to the codend equipped with JTED because it retained substantially fewer under‐sized fish and had the smallest selection range. It therefore seems that replacing the standard codend with a codend equipped with SMP provides the best outcome for the fishery whereby fishermen will increase their catches of larger fish while simultaneously reducing their under‐sized fish catches.     

Growth and mortality of bigeye tuna <Emphasis Type="Italic">Thunnus obesus</Emphasis> (Scombridae) in the eastern and central tropical Pacific Ocean

Biological parameters such as age, growth and age (or size) at maturity are vital for stock assessment and management. Aging is essential in yielding such information. However, limited aging studies have been conducted for large tropical pelagic species in the eastern and central tropical Pacific Ocean. The objective of this study is to conduct a length frequency analysis for estimating growth and mortality of bigeye tuna in the eastern and central tropical Pacific Ocean using samples from the Chinese longline fishery during February to November 2006. The von Bertalanffy growth parameters of asymptotic fork length L _∞ and growth coefficient k were estimated at L _∞ = 207.4 cm fork length, k = 0.23 year^-1, and theoretical age at zero length t ₀ = −0.40 year. The total mortality rate (Z) was estimated to be 0.60; the fishing mortality rate (F) and the natural mortality rate (M) were 0.25 year^-1 and 0.35 year^-1, respectively. The exploitation rate (E) was 0.16. This study provides the estimates of growth and mortality rate for bigeye tuna in the eastern and central tropical Pacific Ocean, which can be used as biological input parameters in further stock evaluations in this region. However, age analysis, further validation of the age composition and stock structure are needed for future studies.     

Reproductive biology of black seabream Acanthopagrus schlegelii,threadfin porgy Evynnis cardinalis and red pargo Pagrus major in the northern South China Sea with consideration of fishery status and management needs

The reproductive biology of three commercially significant seabream species (family: Sparidae) Acanthopagrus schlegelii, Evynnis cardinalis and Pagrus major, taken from Hong Kong and adjacent northern South China Sea (SCS) waters, were investigated for their sexual patterns, spawning seasons, length at maturity and exploitation in relation to their conservation and management status. Histological analysis showed E. cardinalis and P. major to be functionally gonochoristic, the latter having a bisexual juvenile stage and being a rudimentary hermaphrodite. Acanthopagrus schlegelii is a protandric hermaphrodite. Standard length (L_S) at 50% sex change for A. schlegelii is 291 mm. L_S at 50% female maturity for E. cardinalis and P. major are 117 and 332 mm, respectively. For all three species, the spawning period falls between November and March. The study highlights geographical differences in reproductive biology among the species and a paucity of fishery or other population‐related data. While heavy fishing pressure, life‐history characteristics and absence of effective management throughout the geographic ranges of these species make them susceptible to overfishing, they nonetheless appear to be generally more resilient than many other taxa that comprise the multi‐species fisheries of the region, possibly due to their relatively rapid sexual maturation and spatial movement patterns. Overall, however, little is known of the biology, fishing history and current fishery status of sparids in general in the northern SCS and the current study is one of the first to examine such aspects of this family in the region and to consider appropriate management options.     

Gametogenesis, histological gonadal cycle and in vitro fertilization in the whitemouth croaker (Micropogonias furnieri, Desmarest, 1823)

The whitemouth croaker (Micropogonias furnieri) is a demersal teleostean fish inhabiting the Atlantic Ocean from northern Venezuela to southern Argentina. In terms of biomass, the whitemouth croaker is the dominant sciaenid in the Río de la Plata and constitutes an important renewable resource for the Uruguayan and Argentinean fisheries. In the present study, analyzed were the gametogenesis and histological gonadal cycle in female and male whitemouth croakers collected at the Uruguayan coast of the Río de la Plata, between March 1997 and February 1998. In addition, the experimental conditions required to obtain mature viable gametes and achieve in vitro fertilization in this species were investigated. These studies indicate that the whitemouth croaker inhabiting the Río de la Plata is a multiple spawner, which reproduces on the Uruguayan coast between October and February. In vitro studies showed that following activation, sperm remain motile for up to 40 min, and under optimal dilution conditions that they retain a high fertilization capability for 15 min. Taken together these results could support future aquaculture research and exploitation of this species.     

Slow growth of the overexploited milk shark Rhizoprionodon acutus affects its sustainability in West Africa

Age and growth of Rhizoprionodon acutus were estimated from vertebrae age bands. From December 2009 to November 2010, 423 R. acutus between 37 and 112 cm total length (L_T) were sampled along the Senegalese coast. Marginal increment ratio was used to check annual band deposition. Three growth models were adjusted to the length at age and compared using Akaike's information criterion. The Gompertz growth model with estimated size at birth appeared to be the best and resulted in growth parameters of L_∞ = 139·55 (L_T) and K = 0·17 year^?1 for females and L_∞ = 126·52 (L_T) and K = 0·18 year^?1 for males. The largest female and male examined were 8 and 9 years old, but the majority was between 1 and 3 years old. Ages at maturity estimated were 5·8 and 4·8 years for females and males, respectively. These results suggest that R. acutus is a slow‐growing species, which render the species particularly vulnerable to heavy fishery exploitation. The growth parameters estimated in this study are crucial for stock assessments and for demographic analyses to evaluate the sustainability of commercial harvests.     

Growth patterns of three bivalve species targeted by the Ocean Cockle Fishery,southern New South Wales: Eucrassatella kingicola (Lamarck, 1805); Glycymeris grayana (Dunker, 1857); and Callista (Notocallista) kingii (Gray, 1827)

The Ocean Cockle Fishery is a small fishery in southern New South Wales targeting three bivalve species: Eucrassatella kingicola; Glycymeris grayana; and Callista (Notocallista) kingii. The fishery currently consists of a single licensed fisher harvesting a defined patch of seabed but has been identified as potentially able to support expansion. All target species have unknown population dynamics and life histories, and the degree to which expansion can be supported is therefore also unknown. We used internal growth marks in cross sections of shell to determine ages from the commercial catch. The assumption that the alternating bands represent seasonal variations in growth rate was validated by laser ablation induction-coupled plasma mass spectrometry (LA-ICPMS), a small tag-recapture program and preliminary marginal increment analysis. Mean length-at-age was modelled using the von Bertalanffy growth equation. Callista kingii (L_∞?=?43.8, K?=?0.43) grew substantially faster to a smaller maximum size than E. kingicola (L_∞?=?70.0, K?=?0.18) and G. grayana (L_∞?=?54.3, K?=?0.20). These growth characteristics may explain anecdotal observations that C. kingii has increased in relative abundance in recent catches and is relatively rare in unfished areas while E. kingicola in particular has declined in mean size and abundance.     

Fisheries assessment of Chwaka Bay (Zanzibar) – following a holistic approach

This study investigates the typical East African multispecies fishery of Chwaka Bay (Zanzibar) by assessing growth, mortality, exploitation and stock size of six key target species of the fishery. Analyses are based on monthly length–frequency data collected from January to June and September to December in 2014 from the three main landing sites surrounding the bay. Estimated von Bertalanffy growth parameters using the ELEFAN I routine as implemented in FiSAT II varied among species from 27.6 to 64.4 cm (L_∞) and 0.24 to 0.8 per year (K). Current exploitation rates compared to biological reference points calculated from yield‐per‐recruit analysis for five of the six key species indicate that Siganus sutor, Lethrinus borbonicus and Lethrinus lentjan are harvested beyond maximum sustainable levels (E_max). While juvenile retention rates of these three species are >80% and current L_c‐values are lower than L_opt, fishing mortalities for Siganus sutor and Lethrinus borbonicus are highest for specimens above length at first maturity. Two management measures are discussed: (i) an increase in mesh size; and (ii) closure of the destructive dragnet fishery. The first option seems feasible only if the radius of the fishery were increased to capture larger specimens outside the shallow bay area. The second option would leave approximately 550 fishermen unemployed. Reallocating dragnet fishing effort to other gears would lead to a substantial increase in the number of boats, which might create use conflicts over the limited fishing area. The general status of the fishery in Chwaka Bay is considered as representing a ‘full exploitation to over‐exploitation scenario’, with no scope for expansion.     

Age,growth, and age at sexual maturity of the commercially landed skate species,Dipturus chinensis (Basilewsky, 1855), in the northern East China Sea

Age, growth, and age at sexual maturity of the polkadot skate Dipturus chinensis, in the northern East China Sea were determined for a total of 614 specimens collected from April 2009 to December 2014. Vertebral centrum analysis was used to calculate the age of the skates. Annual band deposition was determined by marginal increment analysis. The von Bertalanffy growth model was fitted to the observed length‐at‐age data for each sex (males, L_∞ = 76.8, k = 0.109, t₀ = ?1.28; females, L_∞ = 83.1, k = 0.103, t₀ = ?1.20). Growth patterns of females and males were similar until the age of 6; thereafter, females grew larger than males. Maximum age recorded was 13 years for males and 15 years for females. Age at 50% sexual maturity was 8.22 years for males and 9.39 years for females. These results indicate that D. chinensis is slow growing, relatively long‐lived, and late maturing, and therefore vulnerable to exploitation.     

Age,growth and maturity of the pelagic thresher Alopias pelagicus and the scalloped hammerhead Sphyrna lewini

Indonesia has the greatest reported chondrichthyan catches worldwide, with c.110,000 t caught annually. The pelagic thresher (Alopias pelagicus) and scalloped hammerhead (Sphryna lewini) together comprise about 25% of the total catches of sharks landed in Indonesia. Age and growth parameters were estimated for A. pelagicus and S. lewini from growth‐band counts of thin‐cut vertebral sections. Alopias pelagicus (n = 158) and S. lewini (n = 157) vertebrae were collected from three Indonesian fish markets over a 5 year period. A multi‐model analysis was used to estimate growth parameters for both species. The models of best fit for males and females for A. pelagicus was the three‐parameter logistic (L_∞ = 3169 mm L_T, k = 0·2) and the two‐parameter von Bertalanffy models (L_∞ = 3281 mm L_T, k = 0·12). Age at maturity was calculated to be 10·4 and 13·2 years for males and females, respectively, and these are the oldest estimated for this species. The samples of S. lewini were heavily biased towards females, and the model of best fit for males and females was the three‐parameter Gompertz (L_∞ = 2598 mm L_T, k = 0·15) and the two‐parameter Gompertz (L_∞ = 2896 mm L_T, k= 0·16). Age at maturity was calculated to be 8·9 and 13·2 years for males and females, respectively. Although numerous age and growth studies have previously been undertaken on S. lewini, few studies have been able to obtain adequate samples from all components of the population because adult females, adult males and juveniles often reside in different areas. For the first time, sex bias in this study was towards sexually mature females, which are commonly lacking in previous biological studies on S. lewini. Additionally, some of the oldest aged specimens and highest age at maturity for both species were observed in this study. Both species exhibit slow rates of growth and late age at maturity, highlighting the need for a re‐assessment of the relative resilience of these two globally threatened sharks at current high levels of fishing mortality throughout the eastern Indian Ocean.     

Using cross-dating techniques to validate ages of aurora rockfish (<Emphasis Type="Italic">Sebastes aurora</Emphasis>): estimates of age,growth and female maturity

Since fishery management regulations have shifted much of the groundfish trawl effort in the northeastern Pacific from the continental shelf to the slope, fishery impacts on unassessed demersal slope rockfish species like the aurora rockfish (Sebastes aurora) may have increased. Understanding the life history of these species is a critical first step in developing management strategies to protect them from overharvest. In this study we employ cross-dating methods to validate the annual periodicity of growth increments and investigate the age, growth and maturity of aurora rockfish, a species for which life history information is quite limited. Specimens were collected on an opportunistic basis from Oregon commercial landings and from research cruises, over the years 2003–2006. Age was estimated for 438 individuals using otoliths processed via the break-and-burn method. The maximum estimated age was 118 years for females (n = 324) and 81 years for males (n = 114). The von Bertalanffy growth function showed that males grow faster and reach a smaller maximum size than females (males: L _inf = 34, K = 0.09, t ₀ = −1.9; females: L _inf = 37, K = 0.06, t ₀ = −5.5), though both sexes demonstrate relatively slow growth. Visual assessment of ovaries showed that the aurora rockfish is a synchronous spawner with parturition occurring in May and June off Oregon. Female age and length at 50% maturity were calculated at 12.6 years and 26 cm, respectively (n = 307). Maturity and age data provided evidence for a protracted adolescence in this species.     

Fast versus slow growing tuna species: age,growth, and implications for population dynamics and fisheries management

Growth models describe the change in length or weight as a function of age. Growth curves in tunas can take different forms from relatively simple von Bertalanffy growth curves (Atlantic bluefin, albacore tunas) to more complex two- or three-stanza growth curves (yellowfin, bigeye, skipjack, southern bluefin tunas). We reviewed the growth of the principal market tunas (albacore, bigeye, skipjack, yellowfin and the three bluefin tuna species) in all oceans to ascertain the different growth rates among tuna species and their implications for population productivity and resilience. Tunas are among the fastest-growing of all fishes. Compared to other species, tunas exhibit rapid growth (i.e., relatively high K) and achieve large body sizes (i.e., high L _∞ ). A comparison of their growth functions reveals that tunas have evolved different growth strategies. Tunas attain asymptotic sizes (L _∞ ), ranging from 75 cm FL (skipjack tuna) to 400 cm FL (Atlantic bluefin tuna), and reach L _∞ at different rates (K), varying from 0.95 year^?1 (skipjack tuna) to 0.05 year^?1 (Atlantic bluefin tuna). Skipjack tuna (followed by yellowfin tuna) is considered the “fastest growing” species of all tunas. Growth characteristics have important implications for population dynamics and fisheries management outcomes since tunas, and other fish species, with faster growth rates generally support higher estimates of Maximum Sustainable Yield (MSY) than species with slower growth rates.     

Selectivity of conventional and alternative codend design for five fish species in the Aegean Sea

Currently, traditional Mediterranean trawls are generally made with non‐selective netting and the fishing boats are involved in multi‐species fisheries. As a result, most near‐shore stocks are over‐exploited. Weather permitting, the demersal trawl fleet tends to fish in relatively deeper, international waters of the Aegean Sea, where the catch is usually higher. Therefore, the need for evaluation of the codends used in this fishery and the potential improvements to their selectivity are of prime importance. In the present study, selectivity data were collected for hake (Merluccius merluccius), blue whiting (Micromesistius poutassou), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and fourspotted megrim (Lepidorhombus boscii) in commercial (300 MC) and square mesh top panel (SMTPC) codends. Trawling was carried out at depths of 274–426 m onboard a commercial vessel chartered for a 15‐day sea trial in August 2004. Selection parameters were obtained by fitting a logistic equation using a maximum likelihood method. Results of the selectivity analysis indicated that the commercially used 40 mm nominal mesh size PE codend was rather unselective for the species investigated in this study. In general, the square mesh top panel codend has relatively higher L₅₀ values than the commercial codend. However, except for blue whiting, even this codend is rather unselective when 50% maturity lengths (L_M50) are considered.     

The Gill-Oxygen Limitation Theory and size at maturity/maximum size relationships for salmonid populations occupying flowing waters

The slowing of growth as fish age has long been believed to be related to energy expenditure for maturation, and this rationalization has been used to explain why, across nearly all fish species, the relationship between size at first maturity (L_m) and maximum (L_max) or asymptotic length (L_∞) is relatively constant. In contrast, the Gill-Oxygen Limitation Theory (GOLT) postulates that (a) fish growth slows because as they grow, their two-dimensional ability to extract oxygen from the water diminishes relative to their three-dimensional weight gain, and (b) they can only invest energy for maturation if oxygen supply at their size at first maturity (Q_m) exceeds that needed for maintenance metabolism (Q_∞). It has been reported previously across dozens of marine fish species that the relationship between Q_m and Q_∞ is linear and, further, it can be mathematically converted to L_m vs. L_∞ by raising both terms to the power of D (the gill surface factor), resulting in a slope of 1.36. If the GOLT is universal, a similar slope should exist for L_m^D vs. L_∞^D relationships for freshwater species across multiple individual populations that reside in disparate habitats, although to our knowledge this has never been evaluated. For analysis, we used existing data from previous studies conducted on 51 stream-dwelling populations of redband trout Oncorhynchus mykiss gairdneri, Yellowstone cutthroat trout O. clarkii bouvieri and mountain whitefish Prosopium williamsoni. The resulting L_m^D vs. L_∞^D slopes combining all data points (1.35) or for all species considered separately (range = 1.29–1.40) were indeed equivalent to the slope originally produced for the marine species from which the GOLT-derived relationship was first reported. We briefly discuss select papers both supporting and resisting various aspects of the GOLT, note that it could potentially explain shrinking sizes of marine fish, and call for more concerted research efforts combining laboratory and field expertise in fish growth research.     

Species selectivity in the Eastern Mediterranean demersal trawl fishery using grids to reduce non‐target species

Scientific studies have been conducted on size selectivity of trawls with grids in the Mediterranean; however, none has focused on species selectivity. The aim of the present study was to analyze the value of sorting grids with 20 mm bar spacing for species separation, in a demersal trawl fishery mainly targeting deep water rose shrimp (Parapenaeus longirostris) in the Eastern Mediterranean. Fishing trials were carried out 17–24 August 2008 onboard the commercial trawler ‘Hapulo?lu’, using a modified bottom trawl net. The separation rate for silver scabbard fish (Lepidopus caudatus) was highest among all species with 98% by weight and 97% by number in 44 mm diamond (44D) and 40 mm square mesh (40S) codends, respectively. The exclusion of European hake (Merluccius merluccius) by the grid shows differences between 85 and 93%, in weight and 64 and 83% in number for codends of 44D and 40S, respectively. Excluding the ratio for greater forkbeard (Phycis blennoides), the angler fish (Lophius piscatorius) and broadtail short fin squid (Illex coindetii) varied between 93 and 97% in weight, and 81 and 86% in number in both codends. Losses of legal sized rose shrimp were determined as 23 and 25% in 44D and 40S, respectively. The results obtained in the present study demonstrate that substantial separation of fish and shrimp can be achieved in this Mediterranean demersal trawl fishery. However, further developments should be explored; for this purpose the behaviour of species has to be studied in relation to trawl and grid design and where commercial consequences of changes in gear technology need to be considered.     

Life‐history strategies of the rock hind grouper Epinephelus adscensionis at Ascension Island

Epinephelus adscensionis sampled from Ascension Island, South Atlantic Ocean, exhibits distinct life‐history traits, including larger maximum size and size at sexual maturity than previous studies have demonstrated for this species in other locations. Otolith analysis yielded a maximum estimated age of 25 years, with calculated von Bertalanffy growth parameters of: L_∞ = 55·14, K = 0·19, t₀ = ?0·88. Monthly gonad staging and analysis of gonad‐somatic index (I_G) provide evidence for spawning from July to November with an I_G peak in August (austral winter), during which time somatic growth is also suppressed. Observed patterns of sexual development were supportive of protogyny, although further work is needed to confirm this. Mean size at sexual maturity for females was 28·9 cm total length (L_T; 95% C.I. 27·1–30·7 cm) and no females were found >12 years and 48·0 cm L_T, whereas all confirmed males sampled were mature, >35·1 cm L_T with an age range from 3 to 18 years. The modelled size at which 50% of individuals were male was 41·8 cm (95% C.I. 40·4–43·2 cm). As far as is known, this study represents the first comprehensive investigation into the growth and reproduction of E. adscensionis at its type locality of Ascension Island and suggests that the population may be affected less by fisheries than elsewhere in its range. Nevertheless, improved regulation of the recreational fishery and sustained monitoring of abundance, length frequencies and life‐history parameters are needed to inform long‐term management measures, which could include the creation of marine reserves, size or temporal catch limits and stricter export controls.     

Reproductive strategies of smooth-head catfish Clarias liocephalus (Boulenger, 1898), in the Rwizi-Rufuha wetland system,south-western Uganda

The reproduction of the smooth-head catfish (Clarias liocephalus), a heavily exploited wetland fish in Uganda, in a data-deficient fishery, was studied from January to December 2011. Analyses were based on a sample of 854 fish specimens obtained from a chain of wetlands that fringe the Rwizi River, a tributary of the Nile River. Samples were collected monthly from four sites. Fecundity, gonadosomatic index, size at sexual maturity, condition factor and growth patterns were used to describe the reproduction of the species. Mean female fecundity was 2 484.03 ± 1 289.90 (range 266–3 474). Females attained sexual maturity at a smaller size (12.0 cm TL) than males (13.79 cm TL), but were in better condition than males. Gonadosomatic index peaked in the wettest months of the study period, with highest proportion of mature ova occurring during July to November. Although C. liocephalus seems less fecund than other known clariids and is therefore prone to adverse effects from overexploitation, knowledge of its spawning periodicity and its size at sexual maturity could be useful in the management of the fishery in wetlands where it is still abundant.     

Purification and characterization of chitinase from the stomach of silver croaker Pennahia argentatus

A chitinase was purified from the stomach of a fish, the silver croaker Pennahia argentatus, by ammonium sulfate fractionation and column chromatography using Chitopearl Basic BL-03, CM-Toyopearl 650S, and Butyl-Toyopearl 650S. The molecular mass and isoelectric point were estimated at 42 kDa and 6.7, respectively. The N-terminal amino acid sequence showed a high level of homology with family 18 chitinases. The optimum pH of silver croaker chitinase toward p-nitrophenyl N-acetylchitobioside (pNp-(GlcNAc)₂) and colloidal chitin were observed to be pH 2.5 and 4.0, respectively, while chitinase activity increased about 1.5- to 3-fold with the presence of NaCl. N-Acetylchitooligosaccharide ((GlcNAc)_n, n = 2–6) hydrolysis products and their anomer formation ratios were analyzed by HPLC using a TSK-GEL Amide-80 column. Since the silver croaker chitinase hydrolyzed (GlcNAc)_4–6 and produced (GlcNAc)_2–4, it was judged to be an endo-type chitinase. Meanwhile, an increase in β-anomers was recognized in the hydrolysis products, the same as with family 18 chitinases. This enzyme hydrolyzed (GlcNAc)₅ to produce (GlcNAc)₂ (79.2%) and (GlcNAc)₃ (20.8%). Chitinase activity towards various substrates in the order pNp-(GlcNAc)_n (n = 2–4) was pNp-(GlcNAc)₂ >> pNp-(GlcNAc)₄ > pNp-(GlcNAc)₃. From these results, silver croaker chitinase was judged to be an enzyme that preferentially hydrolyzes the 2nd glycosidic link from the non-reducing end of (GlcNAc)_n. The chitinase also showed wide substrate specificity for degrading α-chitin of shrimp and crab shell and β-chitin of squid pen. This coincides well with the feeding habit of the silver croaker, which feeds mainly on these animals.