Michael J. Fogarty, Rodney Rountree and William J. Michaels
Georges Bank fish communities have undergone profound changes in biomass, productivity, and relative species composition as a result of intensive exploitation. Groundfish populations, including Atlantic cod and haddock, have declined sharply over the last three decades while marked increases in small elasmobranchs (skates and small sharks) have been recorded. Following their depletion by distant water fleets in the 1960's, herring and mackerel populations have rebounded to historically high levels. These changes in system structure provide an important context for interpreting GLOBEC studies on Georges Bank. The target GLOBEC fish species, cod and haddock, are currently at much reduced levels of adult biomass and recruitment while the target copepod species may be subject to much higher levels of predation mortality with elevated populations of planktivorous fishes such as herring and mackerel.Set within broader COP objectives of understanding the determinants of changes within fish communities of Georges Bank, directed studies of the role of predation on the early life stages of cod and haddock by piscivorous fish have been undertaken. These predation process studies are designed to complement GLOBEC research on predation effects on the target organisms by invertebrate predators. Particular attention has been given to potential predation by herring and mackerel on cod and haddock eggs and larvae. Examination of stomach contents of herring and mackerel during cruises conducted during May of 1995 and 1996 documented low-level predation on cod and haddock larvae. In all instances, larval cod and haddock comprised less than 1% (wet weight) of the diet of herring and mackerel. Stomach samples preserved for analysis using DNA probes have not yet been analyzed and may indicate higher incidence of predation on cod and haddock larvae; the larvae are rapidly digested and difficult to visually identify in stomach contents. Even low percentage contribution of cod and haddock larvae to the diets of these pelagic fish predators can translate to substantial removals because of the high biomass of herring and mackerel (estimated to be in excess of 6 million t for both species on a coastwide basis).
Interannual variation in diet composition of herring and mackerel suggest that feeding patterns are opportunistic. The low relative abundance of cod and haddock indicates that encounter probabilities between the predators and cod and haddock larvae are reduced relative to historical levels, contributing to a detection problem which may require large sample sizes to address.
A retrospective analysis of haddock recruitment patterns for the 1961-93 year classes in which a measure of survival rate (loge recruits/spawning biomass) was regressed against herring and mackerel abundance during the first year of life failed to reject the null hypothesis of no effect of herring and mackerel abundance on haddock recruitment (P > 0.05). Inclusion of spring water temperature and temperature-predator interaction terms did not alter this conclusion.
The existing information from analysis of stomach contents and from historical recruitment patterns therefore does not currently support an hypothesis of a controlling influence of predation by these pelagic fish predators on haddock recruitment (subject to the caveats noted above) . Similar analyses are now being undertaken with updated cod recruitment estimates.