Juvenile fish surveys on the Scotian Shelf: Implications for year-class size assessments

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Abstract
The use of juvenile fish surveys for obtaining year-class strength estimates is discussed, using data collected during midwater trawl surveys in the Northwest Atlantic for cod ( Gadus morhua ), haddock ( Melanogrammus aeglefinus ), silver hake ( Merluccius bilinearis ), and herring ( Clupea harengus ). Problems encountered in the development of juvenile fish abundance estimators included changes in availability to survey gear caused by diel or ontogenetic vertical migrations. Data on the vertical and geographical distribution of juvenile fish illustrate the need for careful, speciesspecific survey design based on adequate knowledge of juvenile fish behaviour. For example, a change in methodology of a juvenile silver hake survey on the Scotian Shelf predicated on the results of a study of diel distribution was associated with an increase in the precision of the abundance estimates. While the use of midwater trawl surveys for abundance estimates must be approached with caution, their use in general biological studies of distribution, behaviour, and even stock structure is advocated. In fact, such preliminary investigations are prerequisite to their practical application in juvenile abundance estimation.

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