Gavin Burnell

Food resource, gametogenesis and growth of Mytilus edulis on the shore and in suspended culture: Killary Harbour, Ireland

Mussels, Mytilus edulis L. grow on the shore and are cultured on ropes in Killary Harbour, a fjordic inlet on the Irish west coast. The food resource available to cultured mussels differs from that available to wild mussels on the shore. Although phytoplankton densities as estimated from chlorophyll a concentrations are similar, the shore environment in the inner part of the inlet is characterized by high mean POC concentrations. This is because of the presence of variable amounts of allochthonous detrital carbon. The annual cycles of flesh weight and ash content of wild and cultivated mussels were followed over two years. These cycles were related to the reproductive cycle observed by taking histological samples of mussel gonad, by plankton sampling for larvae and by monitoring larval settlement. Shell growth was measured in wild mussels by reading seasonal growth patterns on sectioned shells and in cultured mussels by following progress of the modal shell length of cohorts on ropes. Wild mussels have a partial spawning in early spring and spawn completely in the summer. Cultured mussels spawn twice during the summer, in the year following settlement. Growth rate of wild mussels decreases with increasing aerial exposure. The fastest growing mussels, at o % exposure, take about 6 years to attain the length attained by the mode of the cultured mussels after 18 months, when they are harvested. We conclude that wild mussels utilize a mix of phytoplankton and detritus as food during the summer and that large wild mussels can use detritus during the autumn and early winter for an increase in flesh weight and gametogenesis. This results in a partial spawning restricted to large individuals in the spring. Cultured mussels are mainly dependent on phytoplankton for food. This supports fast growth and two spawning bouts during the summer, but flesh weight declines once phytoplankton densities fall in the autumn.

Seasonal variation in the chemical composition and fatty acid profile of Pacific oysters (Crassostrea gigas)

Abstract The chemical composition and fatty acid profile of the tray-cultured Pacific oyster (Crassostrea gigas) were measured and compared over a 13 month period. Oysters (120–150 g) were cultured in Cork Harbour and samples taken for analysis at monthly intervals. The moisture, fat, protein, glycogen and ash contents of the flesh were analysed. The fatty acid profile of the flesh was analysed by gas chromatography. The condition indices of the oysters were determined. The chemical composition of the oysters grown in Cork Harbour was similar to literature values for the same species grown in Pacific waters. Ranges for the chemical composition (dry flesh weight basis) were: fat (7.8–8.7%), protein (39.1–53.1%), glycogen (21.6–38.9%) and ash (4.0–12.1%).

Flow-through systems for culturing great scallop larvae

Rearing of great scallop (Pecten maximus L.) larvae was tested in static and flow-through systems in order to improve intensiverearing methods and avoid bacterial problems. The static systems were:standard 300 litre tanks (STD) and standard tanks supplied with anantibacterial agent (STD+A). The flow-through systems were: 500 ldown-welling tanks (FLOW) and a 4700 l up-welling tank (SILO).Bacteral counts ranked the systems in the following order SILO <FLOW < STD+A < STD. In spite of low bacterial numbers, increasedlarval survival was not demonstrated in the FLOW system compared tothe STD system. The highest survival on day 17 (>90%) and day 22(52%) after fertilisation was obtained in the SILO system.The type of rearing system did not affect larval growth.

Reproductive strategies of male and female Loligo forbesi (Cephalopoda: Loliginidae)

The reproductive strategies of male and female Loligo forbesi Steenstrup, 1856 were investigated from samples obtained from commercial catches and research cruises in Irish waters. In females maturity increased with size, but in males two modes in the size at maturity were identified, with approximately 40% mature at small size (180–200 mm mantle length), and the remainder mature at >250 mm mantle length. The difference in estimated age of the two modes of mature males was small, so size differences were probably due to different growth rates. Growth and maturation proceeded together in both sexes over much of the life-cycle. The effect of maturation on relative growth of somatic tissues was examined using analysis of covariance and multivariate regressions. In males there was a significant decline in total mass, and in mass of mantle, head and viscera, relative to mantle length with maturation. In females total mass was not significantly affected by maturation, but relative masses of head, mantle and viscera declined with maturation, indicating that energy was diverted from somatic growth to gonad production. Potential fecundity estimates were obtained by counting eggs and ova in the oviduct and ovary of mature females and were in the range 2500 to 10,500 (mean 5800). Fecundity was positively related to mantle length. The ovaries of mature females contained a range of egg sizes and developmental stages, indicating that spawning probably occurs intermittently.

Age and growth of the squid Loligo forbesi (Cephalopoda: Loliginidae) in Irish waters

Samples of the squid Loligo forbesi Steenstrup 1856 were obtained from commercial catches and research cruises in the Irish and Celtic Seas from August 1991 until October 1993. Age and growth of L. forbesi were estimated from putative daily statolith growth increment counts and from length-frequency data. Indirect evidence of the daily deposition of growth increments was obtained by counting increments on statoliths from immature female squid from successive monthly modes, during a four-month period when length-frequency growth estimates were high. Female growth estimates from length-frequency analysis (15–30 mm per month) were slightly lower than statolith-based estimates (30 mm per month). Statolith data indicated that both sexes had a life-span of approximately one year and that males grew faster and attained a larger size than females. In both sexes growth was found to be logarithmic over the size range sampled (28–505 mm mantle length). Mean estimated age of mature males and females was 317 and 312 days respectively, with the minimum age at maturity found to be 236 and 241 days. Back-calculations of hatching dates showed an extended spawning season from November to May. Squid hatched in the spring grew faster than those hatched in the autumn and winter. In post-recruit L. forbesi, growth of head, mantle and viscera were approximately isometric with body mass. The digestive gland showed slight positive allometry, whilst reproductive organs showed strong positive allometry.

Aquaculture and its role in Integrated Coastal Zone Management

The workshop, “Aquaculture and its role inIntegrated Coastal Zone Management” held inOstende, Belgium in April 2001, aimed to provideinformation to help develop recommendations forwise management of aquaculture in coastal andmarine environments. This paper focuses on thetwo main objectives of the workshop, 1) to aidthe development of stronger interdisciplinarycommunication between the natural and socialsciences and to improve the utility ofavailable scientific information and knowledge;and 2) to promote new perspectives on thesustainable use of coastal areas and naturalresources. The paper also summarizes thedebates following each of the three workshopthematic sessions: aquaculture and ICZM;socio-economic and political dimensions;developing sustainable production systems; andidentifies the key recommendations that may beapplied to the formulation of policy onintegrated aquaculture management in coastaland marine environments.

Recruitment, maturation, and spawning of Loligo forbesi Steenstrup (Cephalopoda: Loliginidae) in Irish waters

A number of reproductive indices were compared with a subjective maturity scale for assessment of Loligo forbesi maturity. The ratio between nidamental gland length and mantle length corresponded well with female maturation, as did the ovary mass-soma mass and nidamental gland mass-soma mass ratios. For males, the ratio between spermatophoric complex mass and somatic mass was found to be the most suitable for maturity assessment. The timing of recruitment and maturation of L. forbesi in Irish waters was described from the size and maturity of squid in commercial landings in the south of Ireland during the years 1991–1993. Immature squid first appeared in commercial catches in July and August, and this represented the main period of recruitment. A second period of recruitment was apparent in December 1991, but was not identified in the 1992–1993 season. Mature females were present in the commercially exploited population between November and April, with a small number also found in the summer. The abundance of egg masses was used to indicate timing of spawning. Egg masses recovered from the Cork coast indicated that peak spawning occurred during the winter months, but continued on a small scale for much of the year.

Bivalve production and food chain efficiency in an experimental nursery system

An experiment was performed to assess the productivity, food chain efficiency and nitrogen conversion efficiency of a semi-closed nursery system for bivalve spat held in upwelling columns. Algal blooms were encouraged by nutrient enrichment of onshore tanks and maintained for short periods under grazing pressure from bivalves.

The effect of stocking density in suspended culture on growth and carbohydrate content of the adductor muscle in two populations of the scallop (Pecten maximus L.) in Bantry Bay, Ireland

The effect of stocking density of the scallop (Pecten maximus L.) in suspended culture using pearl and lantern nets on growth and carbohydrate content of the adductor muscle was assessed in two populations from Mulroy Bay and Bantry in Bantry Bay for 1 year. The results showed that in all treatments, the growth rate increased significantly from June to September. In general, the carbohydrate content in the striated muscle decreased from maximum levels in September to a minimum in March. The carbohydrate content of the smooth muscle was lower than the striated and gradually increased throughout the experiment. The scallops from Bantry Bay had a significantly higher growth and carbohydrate content than the spat from Mulroy Bay. Spat cultured in lantern nets had a significantly higher growth rate than those cultured in pearl nets. In addition, spat cultured at low densities had a higher growth rate and carbohydrate content during the summer than those cultured at high densities.

The diet and ecological importance of Illex coindetii and Todaropsis eblanae (Cephalopoda: Ommastrephidae) in Irish waters

Between November 1993 and May 1997, the stomach contents of trawl-caught Illex coindetii and Todaropsis eblanae from the edge of the continental shelf west and south-west of Ireland were examined. When visually assessed, 82.1% of the I. coindetii stomachs and 73.4% of the T. eblanae stomachs had no food remains present. In both I. coindetii and T. eblanae stomachs with prey, crustaceans, particularly euphausids, are relatively more important as prey in smaller squid. As the squid grow, fish and squid become increasingly important prey. The small mesopelagic fish Maurolicus muelleri is a particularly important prey item of both species, occurring in 48.3% of I. coindetii and 34.7% of T. eblanae stomachs with prey remains. M. muelleri otoliths were also often found in high numbers. In one I. coindetii stomach, 43 pairs of otoliths were found. However, this great abundance may be an over-representation of their importance in the diet as a result of the techniques used to identify prey remains in squid stomachs. Three other pelagic fish, Gadiculus argenteus, Micromesistius poutassou, Argentina sp. and the euphausid Meganyctiphanes norvegica, are also important prey of both species. Data on possible trawlnet-associated feeding are also presented, although the importance of this as a source of error in diet studies is unclear. Using data from research vessel surveys, there seems to be a peak in feeding intensity in the early morning in I. coindetti.