Determining an effective density regime for rearing juvenileAustropotamobius pallipesin a small-scale closed system hatchery

Abstract

With recent advances in aquaculture techniques, captive-breeding of the endangered white-clawed crayfish Austropotamobius pallipes for restocking is becoming a widespread conservation method. Establishing optimal stocking densities for aquaculture is essential in maximizing productivity, and increases the likelihood of crayfish survival when released. A 240-day experiment took place using 2-month-old juvenile, captive-born, A. pallipes, within a small-scale, closed-circuit hatchery to investigate survival, growth and aggression at three treatment densities, low (100/ m 2 ), medium (200/m 2 ) and high (300/m 2 ). Crayfish were counted and measured every 60 days between August 2015 and April 2016. Mean survival rates were high across all three densities (87.7% ± 2.8%). Carapace length was significantly longer at low density than at medium and high densities. While growth rate was not significantly different between treatments, it was significantly higher in the first 2 months, across all three treatments (47.1% ± 6.6%) than in subsequent periods (14.1% ± 5.8%). Size variation within groups increased with density, suggesting that social dominance hierarchies are established with increasing stocking density: dominant individuals are larger and competitively exclude smaller individuals from food resources. Males were significantly larger than females from 6 months of age, (when they could be reliably sexed), in all three treatments. The larger male size suggests that sexual dimorphism begins prior to sexual maturity, with males growing faster and being more dominant and aggressive than females. In conclusion, young-of-the-year A. pallipes can be reared at high densities without compromising survival; however, the optimal stocking density that maximizes growth and health is 100/m 2 .

Effects of Different Diet Types on Growth and Survival of White-Clawed Crayfish Austropotamobius pallipes in Hatcheries

“…This was despite significantly lower numbers of crayfish within the cyst treatment groups during this time period due to mortalities. Previous studies have shown there is an increase in growth with a reduction in crayfish density (Nightingale et al, 2018), which was not observed in this case, and therefore suggests that diet was a limiting factor. Growth of the hatchlings (experiment B) was also significantly greater when fed live Artemia nauplii plus plankton diet (Live + P) than when they were fed the Live + NZ or Spanish diets.…”

“…The reduction in A. pallipes growth experienced with several of the treatment diets from day 40-80 is typical for this species. In a recent paper by Nightingale et al (2018), a 240-day study of A. pallipes hatchlings, found growth was highest in the first 60-days (47.1% ± 6.6%). Growth then dropped to an average 14.1% ± 5.8% increase with every 60-day increment.…”

“…All diets, (except the pellet-only Spanish practical diet treatment, tested in experiments B and D), were enriched with 1 mL multivitamins, 1 g Spirulina, 1 mL lipids, plus 1 g of the carotenoid Astaxanthin, which was added to prevent the crayfish turning blue (Menasveta et al, 1993;Lorenz and Cysewski, 2000). As these enrichments were already included in the Spanish practical diet they were excluded.…”

“…The crayfish may have been attacking moulting crayfish due to a nutritional deficiency within the diet (synchronised moulting was not occurring and therefore some crayfish were susceptible to cannibalism at this time). Alternatively, the reduced numbers of crayfish in the treatment tanks, due to the lower survival rate, caused an increase in growth of the remaining individuals (Savolainen et al, 2004;Nightingale et al, 2018).…”