Egg and larval development in pollack (Pollachius pollachius)
Also known in the UK as Lythe, pollack is a gadoid and is thus closely related to the cod, whiting and haddock. In the wild, females mature at 5-6 years old, and spawn in late winter and spring. A single female around 75 cm length can produce nearly 3 million eggs which are buoyant and float in the plankton. The young fish can be found in shallow inshore waters where there is shelter and good feeding, with their main diet at this stage being crustaceans. Adults live either pelagically or close to the bottom, and can be found in coastal waters down to depths of 200 m. and feed on deep-sea prawns, sand-eels, herring and sprats. Pollack routinely reach 50 cm in length but can grow to 1.3 metres and a weight of 18 kgs .
The total annual European catch of pollack is 14,000 tons, of which France lands 3,500 tons, with Norway and Spain landing 2,500 tons each. Although the market for pollack, and thus its potential for culture, is limited in the UK this study offers the opportunity to determine basic reproductive and rearing parameters for an important gadoid species which is of significant farming interest in countries such as Spain and France.
The purpose of the project is to assess the spawning characteristics of pollack, including an analysis of the variation in both egg quantity and quality throughout the season, and to describe the early stages of egg and larval development of this species. Added to this, as is a very limited published literature on the reproduction and early life history of pollack, the production of cultured juveniles as a result of this research will provide a ready source material for biologists interested in further scientific study of this species.
Ardtoe holds a breeding population of pollack developed from local wild caught fish. This stock are held in a 25m3 tank supplied with ambient temperature water, and fed twice a week on a moist pellet. Total daily egg production and egg quality was measured daily throughout the spawning season, which extended from Eggs were stocked in incubators for 8-9 days at 8C and the different batches transferred to 100 ℓ tanks 2 days before hatching. The resulting larvae were fed on enriched rotifers for the first days and Artemia thereafter. Blastomere morphology of eggs, and then morphometric measurements of eggs and larvae, were recorded throughout the rearing period. The data presented here summarises the results recorded to date
POLLACK COD
Eggs per gram 1,000 (936-1226) 585 (445-775)
Degree day to hatch 87.8 (83-95) 100 (96-101)
Egg diameter (mm) 1.175 (1.1-1.275) 1.4 (1.2-1.5)
Egg wt (mg) 0.083 (0.074-0.096) 0.134 (0.110-0.164)
Fertilisation (%) 65.46 88.6
Hatch length (mm) 3.62 (3.03-4.00) 4.42 (4.05-4.63)
Hatch wt (mg) 0.15 (0.14-0.29) 0.17 (0.06-0.65)
Survival to hatch (%) 92.6 (88.8-96.3) 97 (93-100)
Mouth opening 115 deg days 100-105 deg days
Length 14 dph (mm) 5.51 (5.00-5.86) 5.75 (5.00-6.72)
Future work will focus on refining protocols for larval rearing and weaning, with growth rates and survival being measured and compared with other reared gadoids. Some of the fish will be on-grown to harvest size in order to develop an initial protocol and economic model for their commercial culture.
