A 6-year study of Thelohania contejeani parasitism of the noble crayfish, Astacus astacus L., in Lake Steinsfjorden, S. E. Norway

Abstract. During 1981-1986, the mean prevalence of Thelohania contejeani parasitism of the noble crayfish, Astacus astacus L., in Lake Steinsfjorden, S. E. Norway, was 0.29%. The crayfish population is heavily exploited. Thelohania contejeani infection levels increased significantly during the fishing season, and variations between years were more pronounced in catches after the fishing season (0.38-2.11%) compared to catches before the fishing season (0.05-0-19%). Sixty-eight per cent of the variation between years could be explained from variations in trap effort. Scuba-diving yielded significantly higher T. contejeani infection levels than simultaneous trapping with baited traps. Thus, in years with heavy exploitation, a larger fraction of non-infected crayfish are removed from the population as these are more susceptible to capture. The findings do not support the hypothesis that density and cannibalism influence T. contejeani infection.     

Evaluation of Trap Density and Type for Harvesting Crawfish Procambarus spp. from Small Ponds

Harvested crawfish were stocked into 0.15-ha earthen ponds in March and May at rates of 114, 227, and 455 kg/ha to evaluate the effects of trap density, 27 and 54 traps/ha, on their recapture over 14 d following 3 d of acclimation. Two-funnel, stand-up pillow traps and three-funnel pyramid traps were compared. About 39% and 55% of stocked crawfish were recovered in March and May, respectively. There was considerable growth of those crawfish. Overall catch with both trap densities was equivalent in March but the higher trap density caught 1.3 times more crawfish in May. Pyramid traps caught 1.5–1.9 times more crawfish than pillow traps.     

Mating and spawning of freshwater crayfish, Austropotamobius pallipes Lereboullet, under laboratory conditions

Abstract. The study includes mating and spawning data of freshwater crayfish, Austropotamobius pallipes Lereboullet, obtained in four consecutive years (1987–1990). A total number of 586 wild-caught female A. pallipes in the 21·65 mm to 45·60mm carapace length (CL) size were held under laboratory conditions at different dates in September–October. Initial densities at mating time were 21 and 24 crayfish/m² with a 1:2 male:female ratio. High percentages of mating were obtained (97·4–100%) except in 1989 (57·5%). In all years, most of the matings (90%) were concentrated in a period of 8–12 days with mean water temperatures between 12°C and 13·5°C. Spawning took place a few days after mating (minimum 2 days, maximum 14 days) at mean water temperatures between 8·2°C and 10·8°C. In the first 3 years, almost 100% of mated females spawned. However, in the fourth year (1990), when the mean size of crayfish was smaller, only 72·4% of mated females spawned. The mean number of pleopodal eggs was 64 (maximum 220, minimum 18). Egg diameter ranged between 2·30 and 3·25 mm (mean value 2·78 mm). Pleopodal egg number was positively correlated with carapace length ( r = 0·72).     

Population ecology of cave armoured catfish, Ancistrus cryptophthalmus Reis 1987, from central Brazil (Siluriformes: Loricariidae)

Abstract –  The population ecology of Ancistrus cryptophthalmus (Reis 1987) was studied by mark–recapture technique in caves from the São Domingos karst area, State of Goiás, northeastern Brazil. Total population sizes estimated for Angélica and Passa Três Caves were 20,000 and 1000 individuals, respectively. Densities around 1.0 individuals per m² in Angélica, Bezerra and São Vicente I Streams, and 0.6 individuals per m² in the smaller Passa Três Stream may be considered high for cavefish standards, as well as for epigean loricariids. As expected for benthic grazers, cave catfish are highly sedentary. The distribution of size classes did not differ among caves and within the same cave throughout the studied dry seasons; on the contrary, the condition factor decreased throughout this period probably because of the progressive depletion of organic matter available as food. Low proportions of mature individuals, low growth rates (average = 0.5 mm month⁻¹) with cases of negative growth and high longevities (8–10 years) point to a precocial lifestyle, typical of troglobitic species.     

Growth, mortality and moulting rate of noble crayfish, Astacus astacus L., juveniles in aquaculture experiments

Growth, mortality, moulting rate and cheliped loss were examined in aquaculture experiments with noble crayfish, Astacus astacus L., juveniles from four different Norwegian populations in the period September 1987-June 1988. The juveniles were hatched indoors in May 1987 from parents wild-caught in 1986. The experiments were performed at 18-19°C in communal rearing units. Mean total length at experiment start was about 20mm in all groups. There were no significant differences in mean size (final mean total length: 32-2-34-8mm). mortality (83a-900%) or cheliped loss (429-700%) among juveniles from the different populations when exposed to similar conditions (food: dry pellets; photoperiod: L:D = 8:16). Among crayfish fed fish and potatoes in addition to dry pellets, and with continuous light conditions, mortality was reduced significantly to about 70%. Continuous light conditions significantly increased mean size (final mean length: 36-1-389mm). Moulting occurred from November throughout the experiment period. Moulting rate was positively correlated to mortality and growth rate (i.e. mean length increase). A positive correlation between mortality and cheliped loss, along with direct observations of cannibalism, indicated that agonistic behaviour and cannibalism were the dominating mortality factors. It is suggested that continuous light conditions reduce agonistic behaviour and thereby decrease mortality.     

Effectiveness of Commercially Formulated and Fish Baits in Trapping Cambarid Crawfish1

In the USA, cambarid crawfishes are harvested commercially with traps baited with fishes, formulated baits, or a combination of the two. Commercially formulated baits and fish baits, done and in combination, were evaluated to assess their relative effectiveness M attractants for crawfish. Traps baited with a combination of fish and formulated bait generally captured the most crawfish. Traps baited with common carp, the most effective natural bait, captured more crawfish than traps baited with skipjack herring, gizzard shad or channel catfish heads. Crawfish harvest from traps baited with several types of commercially formulated baits was equal to that obtained with traps baited with either gizzard shad or skipjack herring. Population density of crawfish and the availability of natural forage did not appear to influence relative effectiveness of either formulated baits or fish baits.     

A Preliminary Study of the Spatial Variation in Growth of Raft-Cultured Blue Mussels Mytilus trossulus in Northern Puget Sound, Washington

Abstract.– The effects of culture rope density (high and low), position (inside, middle. and outside), and depth (1, 3, and 5 m) on growth rates, expressed as mean shell length increase (mm) per day, were quantified in raft-cultured blue mussels Mytilus trossulus in northern Puget Sound during the fall of 1993. Growth rates differed by as much as 20% after the 90-d experimental period, and varied significantly according to culture rope position, but not according to culture rope density or depth. Furthermore, a three-way analysis of variance revealed a significant first-order interaction effect, culture rope density × depth. The significant culture rope position effect was due to the higher growth rate (mean ± SEM) of mussels on outside culture ropes (0.199 ± 0.009 mm/d) compared to inside culture ropes (0.168 ± 0.007 mm/d). The interaction effect was due to the significantly higher growth rates of mussels at I and 3 m (0.197 ± 0.009 and 0.204 ± 0.011 mm/d, respectively) compared to 5 m (0.162 ± 0.014 mm/d) under low culture rope density conditions, and the significantly higher growth rate of mussels at 3 m under low culture rope density conditions (0.204 ± 0.011 mm/d) compared to high culture rope density conditions (0.168 ± 0.012 mm/d). Practical raft management is affected by such variation in growth. Commercial mussel growers may find it necessary to alter their culture methods to insure a more uniform product from seed to harvest.     

Juvenile growth of two tilapia species in lakes Naivasha and Baringo, Kenya

Abstract –  Tilapia species in East African lakes often form the basis of commercial fisheries, with information on their population biology assisting their management. This study utilised otolith microstructure, with validation by modal progression analysis, to provide growth data for two tilapia species in two Kenyan lakes for the first time. Otolith microstructure revealed the growth rate of Oreochromis niloticus baringoensis , a sub-species endemic to Lake Baringo, was fast compared with populations of Oreochromis niloticus ; mean daily growth rates were >0.50 mm·d⁻¹ in fish between 12 and 53 mm total length. Growth of Oreochromis leucostictus in Lake Naivasha was also relatively fast when compared with the growth rates of other tilapia species, with mean daily growth rates of approximately 0.35–0.48 mm·d⁻¹ in fish between 10 and 56 mm total length. Growth rates of both species were well described by the Gompertz model and were corroborated by modal progression analysis that revealed similar daily growth rates in modes of young-of-the-year fish.     

Density-Dependent Pond Growout of Single Year-Class Cohorts of a Freshwater Crayfish Cherux tenuimanus (Smith) to Two Years of Age

Pond growout of Cherax tenuimanus to two years of age provided data for mean crayfish weight, surviving density and distribution of individual sizes at intervals and final harvest for 12 single year-class cohorts. Final mean size (47–156 g) was most significantly related to surviving density (4.5–0.7/m²). A hyperbolic model with a harvest date variable accounted for 89% of the variation in mean weight. The biological basis of this model is that the reciprocal of density, i.e., pond area per crayfish, represents food supply for a bottom-dwelling crayfish. By incorporating the Richards growth function, the hyperbolic model can be more realistically fitted to include the limit to potential size at very low densities. The growth-density relationship developed after an initial period of 300 days of low growth (to ∼ 10% of final size) and high mortality. Then, mean weight increased rapidly and density stabilized. Final cumulative production (1.401–3,922 kg/ha) and harvest biomass (1,110–2,991 kg/ha) both increased linearly with density. Survival of the total amount of crayfish produced was about 75% at harvest. Although cohort size distributions were positively skewed, the arithmetic mean size employed in the model was very closely and linearly related to the geometric mean of the lognormal distribution, The causal basis and assumptions of the growthdensity model are discussed in relation to resource sharing by individuals and the disproportionate influence of size-related dominance behaviour.     

Changes in yellow perch length frequencies and sex ratios following closure of the commercial fishery and reduction in sport bag limits in southern Lake Michigan

Abstract  The closure of the Indiana commercial fishery in Lake Michigan in 1997 provided the opportunity to examine the effects of commercial exploitation on the yellow perch, Perca flavescens (Mitchill), population. Data from annual gillnet and trawl sampling, recreational angler creel surveys and commercial fishing harvest were used to determine whether changes in length–frequency distributions and sex ratios were altered following closure. The proportions of large, adult yellow perch were significantly greater after the closure of the commercial fishery. Overexploitation of fish >200 mm by commercial harvest likely truncated the size distribution, but recreational harvest was comparatively small and did not appear to be a factor influencing size distributions. This post-closure period also had a greater proportion of females in trawl catches, and the mean length of harvested fish increased in the recreational fishery. These findings suggest commercial exploitation influenced yellow perch population dynamics in far southern Lake Michigan.     

A test of crayfish predatory and nonpredatory fish species as bait in crayfish traps

The effectiveness of different crayfish: predatory (pike, Esox lucius L., perch, Perca fluviatilis L.) and nonpredatory [roach, Rutilus rutilus (L.) , and bream, Abramis brama (L.)] fish species as bait for noble crayfish, Astacus astacus L., was tested. Trapping was conducted in two lakes in Norway and Lithuania. Nonpredatory bream and roach were superior as noble crayfish bait compared with predatory perch and pike. In the Norwegian lake bream was also superior to roach. Pike- and perch-baited traps tended to catch larger crayfish than bream- and roach-baited traps. An alarm or avoidance response to different live or dead fish predators (e.g. pike or perch) might exist and could be more pronounced in smaller crayfish, thus explaining that pike and perch as bait catch fewer and larger crayfish. Another explanation could be that the odour of pike and perch flesh is simply less attractive or diffuses more slowly through the water. Selecting the most efficient bait will increase the catch per unit effort (CPUE); in this study there was a four-fold variation in CPUE between the best and worst bait type. In test-fishing/monitoring surveys it is important to standardize the bait type used.     

Signal crayfish (Pacifastacus leniusculus) predation upon Atlantic salmon (Salmo salar) eggs

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Interpopulation comparison of size and age at smolting of white-spotted charr, Salvelinus leucomaenis

Abstract – The relationships between size and age at smolting and juvenile growth rates were examined for 13 populations of white-spotted charr ( Salvelinus leucomaenis ). Mean smolt length (ranging 142–210 mm in fork length) and smolt age (2.4–3.7 years) varied significantly among populations and there was a significant negative relationship between smolt length and age. Mean smolt length was positively correlated with mean specific growth rate from age 0 to 1 year, whereas mean smolt age was negatively correlated with specific growth rate. The present study shows that the freshwater environment conditions operating on juvenile growth are important for moulding the phenotypic characteristics of size and age at smolting.     

Spawning site fidelity and apparent annual survival of walleye (Sander vitreus) differ between a Lake Huron and Lake Erie tributary

Fidelity to spawning habitats can maximise reproductive success of fish by synchronising movements to sites of previous recruitment. To determine the role of reproductive fidelity in structuring walleye Sander vitreus populations in the Laurentian Great Lakes, we used acoustic telemetry combined with Cormack–Jolly–Seber capture–recapture models to estimate spawning site fidelity and apparent annual survival for the Tittabawassee River in Lake Huron and Maumee River in Lake Erie. Walleye in spawning condition were tagged from the Tittabawassee River in Lake Huron and Maumee River in Lake Erie in 2011–2012. Site fidelity and apparent annual survival were estimated from return of individuals to the stream where tagged. Site fidelity estimates were higher in the Tittabawassee River (95%) than the Maumee River (70%) and were not related to sex or fish length at tagging. Apparent annual survival of walleye tagged in the Tittabawassee did not differ among spawning seasons but was higher for female than male walleye and decreased linearly as fish length increased. Apparent annual survival of walleye tagged in the Maumee River did not differ among spawning seasons but was higher for female walleye than male walleye and increased linearly as fish length increased. Greater fidelity of walleye tagged in the Tittabawassee River than walleye tagged in the Maumee River may be related to the close proximity to the Maumee River of other spawning aggregations and multiple spawning sites in Lake Erie. As spawning site fidelity increases, management actions to conserve population structure require an increasing focus on individual stocks.     

Effects of rearing technique and fish size on post-stocking feeding, growth and survival of European grayling, Thymallus thymallus (L.)

European grayling, Thymallus thymallus (L.), were reared by four different techniques: (A) in a natural pond without a supply of artificial diet; (B) in a net pen fed artificial dry diet; (C) in tanks fed artificial dry diet; (D) in tanks, and for 16 days before stocking, transferred to a diet based on mixed frozen natural food exclusively. Fish within groups C and D were also split into subgroups of large and small fish. Feeding, growth and survival of fish in the different groups were compared after stocking into three different stocking sites during the spring of 1994. The stocking sites consisted of a barren lake (Lake L), a second lake (Lake N) containing an allopatric perch, Perca fluviatilis L., population, and a barren man-made pond. In Lake L, higher recapture rates and mean weights of stomach contents were obtained in fish reared in the natural pond and in the net pen. There were no differences in total recapture rate, or mean condition factor between small and large fish within groups C and D. In Lake N only five (0.5%) stocked grayling from rearing groups A and B were recaptured during the experiment and more fish from categories C and D were recaptured. This was probably related to size-selective predation. In the pond, there were no differences in recapture rates or in mean weight of stomach contents between fish from any of the groups. In sample 1 (Lake N, 22 days after stocking), mean weight of stomach contents was higher in the larger fish within categories C and D but there was no difference in condition factor between the groups. In sample 1, mean condition factor of fish from the different groups varied markedly, being highest in Lake L followed by Lake N and the Pond. Fourteen out of 26 dead fish in the pond showed clinical symptoms of the bacterial disease ASA. Mortality was related to low initial condition factor at the time of stocking. No stocked grayling were recaptured from the two lakes in the last sample (June 1995).     

Evaluation of a by-catch reduction device for glass eel fishing traps

Abstract  The effectiveness of a new design of glass eel, Anguilla anguilla (L.), fishing trap was tested in the Ebro River delta fishery (Eastern Mediterranean, Spain). The modified fishing trap incorporated a by-catch reduction device (BRD) to increase trap selectivity and minimise by-catch of non-targeted fish, without affecting glass eel captures. The BRD is inexpensive and easy to implement, consisting of a cylinder covered with a rigid square-mesh of 2.0–2.4 mm knot-to-knot, which is easily fitted to the fishing trap entrance. Testing of the BRD was made by paired comparisons between captures of targeted and non-targeted fish species from both conventional and BRD-modified traps. The use of a BRD in the glass eel traps resulted in a 68.9% reduction in the number of captured non-targeted fish specimens, while it did not reduce glass eel yields. The BRD implementation was fully effective for all species and individuals with sizes >40 mm total length, with the exception of the black-striped pipefish, Syngnathus abaster Risso. These results support the implementation of the BRD in glass eel fisheries along the Eastern coast of the Iberian Peninsula where these fishing traps are used.     

Effects of Water Temperature, Density, and Sex Ratio on the Spawning Rate of Red Claw Crayfish Cherax quadricarinatus (von Martens)

Effects of water temperature on red claw crayfish Cherax quadricarinatus spawning rates was evaluated for 3 mo at 22, 26 and 30 ± 1 C. A 20/m² stocking density and a 1:2 M: F sex ratio was used. Spawning rate increased as temperature increased, with 30 C providing the highest spawning rate ( P < 0.05). Effects of stocking density and sex ratio on the reproduction of red claw crayfish were also evaluated at densities of 10.15 and 20/m², with a sex ratio of 1:1, and at three sex ratios (1:1. 1:3 and 1:5 M: F) at a density 10/m². Water temperature was maintained at 28 ± 1 C and photoperiod was maintained at 14L: 10D for 6 mo during the density and sex ratio experiments. Spawning rates were not significantly different ( P > 0.05) within the three stocking densities at a sex ratio of 1:1, nor were they different ( P > 0.05) for the three sex ratios at a density of 10/m². Results revenled that densities as high as 20/m² and sex ratios of 1 male to 5 females can be used to increase spawning per tank. Also, high temperature (28 ± 1 C) and long day lengths (14L: 10D) can be utilized to stimulate peak spawning rates, but high spawning rates cannot be maintained for more than about 3 mo.     

Effects of density on growth and survival of juvenile Red Swamp Crayfish, Procambarus clarkii (Girard), reared under laboratory conditions

The aim of this study was to quantify the importance of population density on the growth and survival of juvenile red swamp crayfish, Procambarus clarkii (Girard), and to understand the effects of density on both the period between moults and the increment per moult at high densities without food limitation. A laboratory experiment was performed to determine growth and survival of juvenile crayfish at five densities (20, 40, 60, 80 and 100 crayfish m^?2). Density clearly affected crayfish growth. The impact of density on final weight was higher than on total length increase, growth rate and final length (FL) (by decreasing order of effect). When comparing minimum and maximum densities, we observed that FL suffered a 34% reduction. Number of moults, mean intermoult period (IP) and survival were not significantly affected by density. Our results indicated that the IP and the percentage of increment per moult are affected by the size of crayfish before moult and we provide equations for these relations. Our findings are relevant both for crayfish aquaculture management and for the management of wild or harvested rice‐field crayfish populations.     

山东半岛南部海域三疣梭子蟹增殖放流群体回捕率

根据2010年和2011年5-8月在山东半岛南部海域对三疣梭子蟹(Portunus trituberculatus)增殖放流前后分别进行的每年3个航次的对比调查结果,定量分析了三疣梭子蟹自然群体和增殖放流群体的数量,并根据三疣梭子蟹放流苗种数量和放流后捕捞产量,对三疣梭子蟹增殖放流群体回捕率进行分析.研究结果表明,2010年5月山东半岛南部海域三疣梭子蟹增殖放流前自然群体资源量很少,相对资源密度为0.90 ind/(站.h);在5-6月共放流苗种11 015.13万只,放流后10d左右资源量增加为26.08 ind/(站.h),放流群体所占的比例为96.55％;8月进行调查的相对资源密度为1.70 ind/(站.h),当年生群体数量占所有群体数量的比例为64.29％;9月、10月、11月秋汛期间捕捞产量为3 108 t,捕捞个体平均质量为232.3 g,增殖放流群体回捕率为7.54％.201 1年5月山东半岛南部海域三疣梭子蟹增殖放流前自然群体相对资源密度为1.70 ind/(站.h);在5-6月共放流苗种13 132万只,放流后10d左右资源量增加为29.15 ind/(站.h),放流群体所占的比例为94.17％;8月进行调查的相对数量为1.00 ind/(站.h),当年生群体数量占所有群体数量的比例为70.00％;9月、10月、11月秋汛期间捕捞产量为2 896 t,回捕个体平均质量为226.0 g,增殖放流群体回捕率为6.43％.除2011年胶州湾海域由于富营养化水质较差,以及其他因素对黄家塘湾海域的影响导致这两个放流点放流群体回捕率偏低外,2010年和2011年山东半岛南部放流海域整体水质良好,水温、透明度、盐度、营养盐比例、生物饵料等均比较适中,属于放流环境条件较佳的海域,比较适合三疣梭子蟹生长,放流群体回捕率整体上处于较高水平.     

Survival and growth of sardine larvae in the offshore side of the Kuroshio

After 1984 the major spawning of the Japanese sardine, Sardinops melanostictus, has been observed to occur in the offshore waters, where the survival of early-stage larvae (4–10 mm in length) is questionable. The main objective of this study is to estimate the growth and survival rates of the early-stage larval cohort in the offshore side of the Kuroshio Current. A radar-reflecting buoy with a surface drogue was launched to tag a patch of larvae, and the patch was traced for 3 days in March 1991. The survival rate of the early-stage larval cohort was calculated from the change in density during the survey. The range of the instantaneous mortality rate was from 0.83 to 1.11 day, a survival rate of32-44%day-¹.The mean density of the smaller larvae (4–6 mm in length) decreased more rapidly than that of the larger larvae (6–10 mm in length) during the sampling period. The survival rate of the first-feeding larvae in the offshore region seemed to be lower than that of the post-flrst-feeding larvae. However, the growth rates of the first-feeding larvae's survivors in the region were higher than those of the post-first-feeding larvae's survivors. The first-feeding larvae in the offshore region seemed to survive when the growth rate of the larvae was high.