Modelling larval haddock Melanogrammus aeglefinus and cod Gadus morhua densities and mortality rates in hatchery mesocosms

The numbers and instantaneous mortality rate Z of haddock Melanogrammus aeglefinus and cod Gadus morhua larvae in a hatchery from 0 to 50 days post hatch (dph) were estimated directly and non-destructively. A small mesh plankton net was raised vertically in the water column from the tank base to the water surface and larvae were examined and counted. Using this direct sampling method Z was assessed through the larval period. A semi-log plot described the relationship of larval numbers on time. The Z of M. aeglefinus larvae from 0 to 50 dph was consistent between tanks, from 11.2 to 14.8% d−1 in seven rearing tanks sampled, with a common regression coefficient of 12.7%. The Z was lower for the period 10–50 dph, from 5.5 to 11.8% d−1 with a calculated common slope for all tanks giving a Z of 9.6%. Restricted data for G. morhua suggested lower mortality of Z of 7.8% d−1 from 0 to 50 dph with 4.2% solely for the period 10 to 50 dph. Avoidance of the net sampler by larvae was generally not recorded until >50 dph. The Z in both M. aeglefinus and G. morhua larvae was similar to Z calculated for these species in sea fisheries surveys. Advantages of the technique for estimating Z in hatchery mesocosms and examining causes and timing of larval mortality are discussed. Care in sampling larvae without removal from water, minimizing tank disturbance by sampling after tank cleaning, sampling across tank locations, and thorough disinfection of sampling nets are essential. •non-destructive method measured larval numbers in rearing tanks using vertically hauled net•instantaneous mortality (Z) of larval haddock from 11.2–14.8% d−1 from 0 to 50 dph•haddock mortality significantly higher than 7.8% d−1 in cod•high mortality identified in the first 10 days post hatch in both gadoids•haddock Z benchmarked with and similar to wild counterparts