Temporal changes in population density, fecundity, and egg size of the Hawaiian spiny lobster (Panulirus marginatus) at Necker Bank, Northwestern Hawaiian Islands

Fecundity (F, number of brooded eggs) and egg size were estimated for Hawaiian spiny lobster (Panulirus marginatus) at Necker Bank, North-western Hawaiian Islands (NWHI), in June 1999, and compared with previous (1978–81, 1991) estimates. Fecundity in 1999 was best described by the power equations F = 7.995 CL 2.4017, where CL is carapace length in mm (r2=0.900), and F = 5.174 TW 2.758, where TW is tail width in mm (r2=0.889) (both n=40; P< 0.001). Based on a log-linear model ANCOVA, size-specific fecundity in 1999 was 18% greater than in 1991, which in turn was 16% greater than during 1978–81. The additional increase in size-specific fecundity observed in 1999 is interpreted as evidence for further compensatory response to decreased lobster densities and increased per capita food resources that have resulted either from natural cyclic declines in productivity, high levels of harvest by the commercial lobster trap fishery, or both.

The effect of intensive line fishing on the virgin biomass of a tropical deepwater snapper, the crimson jobfish (Pristipomoides filamentosus)

An intensive commercial hook-and-line fishing operation targeted the demersal fisheries resources at Saya de Malha Bank in the Southwest Indian Ocean. Fishing was conducted with 12 dories that were equipped with echo sounders and electric fishing reels and supported by a refrigerated mothership. Over a 13-day period in the 55–130 m depth range, a total of 74.3 metric tons (t) of fish were caught, of which the crimson jobfish (Pristipomoides filamentosus) represented 80%. Catch rates decreased with time and could not be attributed to changes in location, climatic conditions, fishing depth, fishing method, or bait type. The initial virgin biomass of P. filamentosus available to a line fishery at the North Western promontory of Saya de Malha Bank was estimated at 72.6 t through application of the Leslie model to daily catch and effort data. Biomass densities of 2364 kg/km2 and 1206 kg/km were obtained by applying the initial biomass estimates to the surface area and to the length of the dropoff that was fished. The potential sustainable yield prior to exploitation was estimated at 567 kg/km2 per year. The quantity of P. filamentosus caught by the mother-ship-dory fishing operation represented 82% of the initial biomass available to a hook-and-line fishery, equivalent to more that three times the estimated maximum sustainable yield. The results of the study are important to fisheries managers because they demonstrate that intensive line fishing operations have the potential to rapidly deplete demersal fisheries resources.

Reproductive biology of the blue swimmer crab (Portunus pelagicus, Decapoda: Portunidae) in five bodies of water on the west coast of Australia

Portunus pelagicus was collected at regular intervals from two marine embayments and two estuaries on the lower west coast of Australia and from a large embayment located approximately 800 km farther north. The samples were used to obtain data on the reproductive biology of this species in three very different environments. Unlike females, the males show a loosening of the attachment of the abdominal flap to the cephalothorax at a prepubertal rather than a pubertal molt. Males become gonadally mature (spermatophores and seminal fluid present in the medial region of the vas deferentia) at a very similar carapace width (CW) to that at which they achieve morphometric maturity, as reflected by a change in the relative size of the largest cheliped. Logistic curves, derived from the prevalence of mature male P. pelagicus, generally had wider confidence limits with morphometric than with gonadal data. This presumably reflects the fact that the morphometric (allometric) method of classifying a male P. pelagicus as mature employs probabilities and is thus indirect, whereas gonadal structure allows a mature male to be readily identified. However, the very close correspondence between the CW50’s derived for P. pelagicus by the two methods implies that either method can be used for management purposes. Portunus pelagicus attained maturity at a significantly greater size in the large embayment than in the four more southern bodies of water, where water temperatures were lower and the densities of crabs and fishing pressure were greater. As a result of the emigration of mature female P. pelagicus from estuaries, the CW50’s derived by using the prevalence of mature females in estuaries represent overestimates for those populations as a whole. Estimates of the number of egg batches produced in a spawning season ranged from one in small crabs to three in large crabs. These data, together with the batch fecundities of different size crabs, indicate that the estimated number of eggs produced by P. pelagicus during the spawning season ranges from about 78,000 in small crabs (CW=80 mm) to about 1,000,000 in large crabs (CW=180 mm).

Reproduction and recruitment of white mullet (Mugil curema) to a tropical lagoon (Margarita Island, Venezuela) as revealed by otolith microstructure

The reproductive activity and recruitment of white mullet (Mugil curema) was determined by observations of gonad development and coastal juvenile abundance from March 1992 to July 1993. Adults were collected from commercial catches at three sites in northeastern Venezuelan waters. Spawning time was determined from the observation of macroscopic gonadal stages. Coastal recruitment was determined from fish samples collected biweekly by seining in La Restinga Lagoon, Margarita Island, Venezuela. The examination of daily growth rings on the otoliths of coastal recruits was used to determine their birth date and estimate the period of successful spawning. Fish with mature gonads were present throughout the year but were less frequent between September and January when spawning individuals migrated offshore. In both years, juvenile recruitment to the lagoon was highest between March and June when high densities of 25–35 mm juveniles were observed. Back-calculated hatching-date frequency distributions revealed maximum levels of successful spawning in December–January that were significantly correlated with periods of enhanced upwelling. The relation between the timing of successful spawning and the intensity of coastal recruitment in white mullet was likely due to variations in food availability for first-feeding larvae as well as to variations in the duration of the transport of larvae shoreward as a result of varying current conditions associated with upwelling.

Distribution, demography, and discard mortality of crabs caught as bycatch in an experimental pot fishery for toothfish (Dissostichus eleginoides) in the South Atlantic

Between March 2000 and April 2001 two commercial fishing vessels fished for toothfish (Dissostichus eleginoides) off South Georgia using pots. A significant number of lithodid crabs (three species of Paralomis spp.) were caught as bycatch. Paralomis spinosissima occurred in shallow water, generally shallower than 700 m. Paralomis anamerae, not previously reported from this area and therefore representing a considerable southerly extension in the reported geographic range of this species, had an intermediate depth distribution from 400 to 800 m. Paralomis formosa was present in shallow waters but reached much higher catch levels (and, presumably, densities) between 800 and 1400 m. Differences were also noted in depth distribution of the sexes and size of crabs. Depth, soak time, and area were found to significantly influence crab catch rates. Few crabs (3% of P. spinosissima and 7% of P. formosa) were males above the legal size limit and could therefore be retained. All other crabs were discarded. Most crabs (>99% of P. formosa, >97% of P. spinosissima, and >90% of P. anamerae) were lively on arrival on deck and at subsequent discard. Mortality rates estimated from re-immersion experiments indicated that on the vessel where pots were emptied directly onto the factory conveyor belt 78–89% of crabs would survive discarding, whereas on the vessel where crabs were emptied down a vertical chute prior to being sorted, survivorship was 38–58%. Of the three, P. anamerae was the most vulnerable to handling onboard and sub-sequent discarding. Paralomis spinosissima seemed more vulnerable than P. formosa.

Larval and settlement periods of the northern searobin (Prionotus carolinus) and the striped searobin (P. evolans)

This study reports new information about searobin (Prionotus spp.) early life history from samples collected with a Tucker trawl (for planktonic stages) and a beam trawl (for newly settled fish) from the coastal waters of New Jersey. Northern searobin, Prionotus carolinus, were much more numerous than striped searobin, P. evolans, often by an order of magnitude. Larval Prionotus were collected during the period July–October and their densities peaked during September. For both species, notochord flexion was complete at 6–7 mm standard length (SL) and individuals settled at 8–9 mm SL. Flexion occurred as early as 13 days after hatching and settlement occurred as late as 25 days after hatching, according to ages estimated from sagittal microincrements. Both species settled directly in continental shelf habitats without evidence of delayed metamorphosis. Spawning, larval dispersal, or settlement may have occurred within certain estuaries, particularly for P. evolans; thus collections from shelf areas alone do not permit estimates of total larval production or settlement rates. Reproductive seasonality of P. carolinus and P. evolans may vary with respect to latitude and coastal depth. In this study, hatching dates and sizes of age-0 P. carolinus varied with respect to depth or distance from the New Jersey shore. Older and larger age-0 individuals were found in deeper waters. These variations in searobin age and size appear to be the combined result of intraspecific variations in searobin reproductive seasonality and the limited capability of searobin eggs and larvae to disperse.

Effects of experimental harvest on red sea urchins (Strongylocentrotus franciscanus) in northern Washington

Commercial harvest of red sea urchins began in Washington state in 1971. Harvests peaked in the late 1980s and have since declined substantially in Washington and other areas of the U.S. west coast. We studied effects of experimental harvest on red sea urchins in San Juan Channel (SJC), a marine reserve in northern Washing-ton. We recorded changes in density and size distribution of sea urchin populations resulting from three levels of experimental harvest: 1) annual size-selective harvest (simulating cur-rent commercial urchin harvest regulations), 2) monthly complete (non–size selective) harvest, and 3) no harvest (control) sites. We also examined re-colonization rates of harvested sites. The red sea urchin population in SJC is composed of an accumulation of large, old individuals. Juvenile urchins represent less than 1% of the population. Lower and upper size limits for commercial harvest protect 5% and 45% of the population, respectively. Complete harvest reduced sea urchin densities by 95%. Annual size-selective harvest significantly decreased sea urchin densities by 67% in the first year and by 47% in the second year. Two years of size-selective harvest significantly altered the size distribution of urchins, decreasing the density of legal-size urchins. Recolonization of harvested sites varied seasonally and occurred primarily through immigration of adults. Selective harvest sites were recolonized to 51% and 38% of original densities, respectively, six months after the first and second annual harvests. Yields declined substantially in the second year of size-selective harvest because of the fishing down of the population and because of low recolonization rates of harvested sites. We recommend that managers consider the potential efficacy of marine harvest refuges and reevaluate the existing upper and lower size limits for commercial harvest to improve long-term management of the sea urchin fishery in Washington.

Early and better maturity in carp brood stock with control of environment and feeding

Available of carp breeders in their prim state of maturity is a major constraint in hypophysation. Experiments conducted in a fish farm at Naihati, West Bengal, for two consecutive years, 1983-84 and 1984-85, clearly prove that by manipulation of environmental parameters such as metabolites, dissolved oxygen, running water conditions, as also of stock densities and quality and quantity of feed. Catla catla, Hypophythalmichthys molitrix, Labeo rohita, Cirrhina mrigala and Ctenopharyngodon idella can be made to attain better maturity and spawning stage much earlier than normal i.e. even in summer months and the entire stock spawned during the period from March to September. Percentage of successful breeding, quantities of eggs released and fertilised in relation to the body weight of all the species, were also found to be more in comparison to the brood stock raised through the conventional methods.

On the response of zooplankton in an electrical field

Zooplankton showed directional movement towards the positive electrode in an electrical field till the current continued to flow. Examination of the plankton 24 hours after their exposure for one minute to different current densities showed considerable mortality, the percentage of which was more in the case of the larger size groups.

On the effect of electrical field on Indian carps in captivity

Threshold current densities required for narcosis of fish in captivity differed in different species. Increased temperature of the water had greater effect on the fishes while changes of pH had no appreciable effect on narcosis in the fishes. Impulse currents of 50 V were found to have better effect on the fishes than continuous current of 180-220 V.

Nursery rearing of Macrobrachium rosenbergii (de Man) using hapa-nets: effects of stocking density

An experiment was conducted in two phases for 45 days each to study the effects of six stocking densities (phase-I: 100, 200 and 300 PL/m² and phase-II: 250, 500 and 750 PL/m²) on growth and survival of Macrobrachium rosenbergii postlarvae (PL) in nylon hapa-nets (1.8m x 1.8m x 1.4m) installed in a pond. Stocking densities of 100, 200 and 300 PL 10/m² resulted in similar (P<0.05) body length (47-48 mm) and survival rate (84-88%), while body weight (0.62g) in PL with 300/m² was significantly lower than that (0.70g) in PL with 100 and 200/m². The growth (body length 47mm and weight 0.64g) and survival (84%) of PL stocked at 250 PL/m² density were significantly higher (P<0.05) than that of PL at 500 and 750/m². Besides the variation in growth and survival in PL at six tests stocking densities, a sharp increase in body weight of PL was observed beginning at the 4th week of rearing.

Impact of stocking density on growth and survival rate of mud crab (Scylla serrata Froskal)

A 10-weeks culture trial of mud crab, Scylla serrata in brackish water earthen pond was conducted in different stocking densities. The aim of the experiment was to identify a suitable stocking density for optimum production. There were three treatment as 5000 crab lings/ha, 10000 crab lings/ha and 15000 crab lings/ha of each with three replications. The initial mean weight of crab lings were same (5.5 ± 0.13 g). The experimental month was June '95 to August '95. The size of each pond was 500 m². To maintain good water quality water was exchanged in every spring tide. The salinity during the experiment were 2-18ppt. Prepared feed of about 32% protein consisting fish meal, MOC rice bran and wheat flour was used at 5% of their body weight. In terms of production, survival rate, growth and carapace width, the stocking density having 10000/ha showed the best (P

Limnological conditions of naturally turbid ponds and its effects on fish growth

An experiment was conducted on six naturally turbid ponds in the village of Salakandi situated to the south-west corner of Bangladesh Agricultural University, Mymensingh for a period of three months from July to September '96. The experiment was performed by studying the physicochemical factors of water and soil, the biological factors such as densities of phytoplankton and zooplankton, and the growth of fishes. During the experimental period water turbidity varied among ponds. The highest value of turbidity was found to be 679±183.6 FTU in pond 1 and the lowest was 158±23.31 FTU in pond 4. The maximum and the minimum water transparency were recorded in the month of July and September respectably. The lowest net weight of fishes was found 28.8 kg/ha/year in pond 1 due to high turbidity and the highest was 35.8 kg/ha/year in pond 4 due to low turbidity. Most of the correlations between turbidity and transparency, phytoplankton, and zooplankton were significant at 1% and 5% levels.

Effect of stocking density on the growth of Thai pangas, Pangasius sutchi (Fowler) in net cage fed on formulated diet

A three month long experiment was conducted to observe the effect of stocking density on the growth of Pangasius sutchi in net cages. The size of each cage was 1m³.The three stocking densities used were 40, 50 and 60 fishes/m³ and designated as treatment T1, T2 and T3 respectively. Each treatment had three replicates. All the fishes were of same age group having mean length and weight of 7.13 ± 1.37 cm and 2.46 ± 0.12 g respectively. The fish in all the net cages were fed a diet containing 34% protein. The result of the study showed that fish in the treatment T1 stocked at the rate of 40 fish/m³ resulted the best individual weight gain followed by T2 and T3 respectively. The specific growth rate (SGR) ranged between 3.51 and 3.09, the food conversion ratio (FCR) values ranged between 1.73 and 2.04 with treatment T1 resulting the lowest FCR. The protein efficiency ratios (PER) values were 1.69, 1.16 and 1.43 for treatment T1, T2 and T3 respectively. There was no significant (P>0.05) variation among the survival rates of fish which ranged between 92 and 95%. The net productions in different treatments were 2189, 2343, and 2283g for treatment T1, T2 and T3 respectively. The result of the present study indicated that the best individual growth of P. sutchi was obtained at a density of 40 fish/m³ but the highest total production was obtained at a stocking density of 50 fish/m³ in net cages.

Minimization of cannibalism of African catfish (Clarias gariepinus Burchell) larvae in indoor culture system

For minimizing cannibalism of African catfish (Clarias gariepinus) larvae two trials for a period of 14 and 15 days respectively in four aquaria of size 120x49x32 cm³ were conducted. Seven days old African catfish larvae with an initial total length and weight of 7.84 (±0.40) mm and 4.40 (±1.18) mg respectively in the first trial and similarly 7.52(±0.61) mm and 3.98 (±0.56) mg in the second trial at the rate of same stocking densities of 2500 larvae in each aquarium were stocked in both trials. Cannibalistic larvae were separated by using grader frame from each treatment at 7 days and 5 days interval during first and second trial respectively. Two mesh sizes i.e., 5 mm and 7 mm were used in the grader frame in both trials. Survival rate was significantly higher in T1 than that of T2 in each trial. Grading of larvae with 5 days interval resulted higher survival rate than that of 7 days interval.