Publisher Summary This chapter discusses the field investigation of the early life stages of marine fish. The motivations for investigations have changed little over the intervening period, being mainly the assessment of adult spawning biomass and distribution, and the desire to understand how environmental variations and changes in the abundance of other species interact to regulate the abundance in particular fish populations. In many cases, the annual recruitment of new juveniles into the population is the most important process governing population fluctuations. The factors affecting recruitment, particularly those affecting the survival of the eggs and larvae, are therefore perceived to be of key importance. In the review of the chapter, the progress of field research on the early life stages of marine fin fish is examined. The objectives of field studies are briefly stated in the chapter, followed by a review of sampling methodology, analytical techniques, and the methods for assessing adult spawning biomass from egg and larval abundance data. This chapter also discusses the field evidence for variability in the growth, survival, and behavior in relation to biological and physical oceanographic conditions concentrating on larvae but including eggs were relevant. Finally, the match-mismatch and member-vagrant hypotheses, and the role of density-dependence in early life stage regulation, are considered in the light of the field evidence.

Field Investigations of the Early Life Stages of Marine Fish

A quantitative analysis of the food intake of North Sea cod, Gadus Morhua

The signs and strengths of spatial correlations between densities of predators and their prey were scale dependent both in simulations and when calculated from acoustic transects across nearshore distributions of Atlantic cod (Gadus morhua), and its prey, capelin (Mallotus villosus). Spectral analyses of 1985 and 1986 transects indicated that spatial correlation (coherence) varied relative to: (1) the occupation of thermal refuge areas by capelin (sea temperatures 8.5°C), (2) the aggregation dimensions of each species, and (3) the activity level of the predator. When capelin did not occupy thermal refuges, cod and capelin densities were coherent across a wide range of scales (2—30 km). At large scales (>4—10 km) densities were in phase (positively correlated), but at scales less than aggregation dimensions (3.5 km), densities were out of phase (negatively correlated). At the smallest scales measured (2—3 km), densities exhibited maximum negative correlation in both years. When capelin were aggr...

The importance of scale to predator-prey spatial correlations: an example of Atlantic fishes

The signs and strengths of spatial correlations between densities of predators and their prey were scale dependent both in simulations and when calculated from acoustic transects across nearshore distributions of Atlantic cod (Gadus morhua), and its prey, capelin (Mallotus villosus). Spectral analyses of 1985 and 1986 transects indicated that spatial correlation (coherence) varied relative to: (1) the occupation of thermal refuge areas by capelin (sea temperatures 8.50C), (2) the aggregation dimensions of each species, and (3) the activity level of the predator. When capelin did not occupy thermal refuges, cod and capelin densities were coherent across a wide range of scales (2-30 km). At large scales (>4-10 km) densities were in phase (positively correlated), but at scales less than aggregation dimensions (3-5 km), densities were out of phase (negatively corre- lated). At the smallest scales measured (2-3 km), densities exhibited maximum negative correlation in both years. When capelin were aggregated within thermal refuges, predator- prey densities were coherent (in phase) only at scales larger than those of the refuges. When actually feeding, cod achieved coherent, in-phase distributions with capelin at scales as small as 3.5 m.

The importance of scale to predator-prey spatial correlations: an example

Stomach content data collected from cod caught during offshore resource assessment surveys in three Arcto-boreal ecosystems (the Barents Sea and the shelves off Iceland and eastern Newfoundland) were examined to determine the prevalence of cannibalism, and to identify possible factors associated with variability in prevalence. Cannibalism provides a minor source of food, except for the largest predators. Cannibalism Increases with predator length. Most prey are less than 40 cm In length and less than 3 years old. In each region, cannibalism occurs over most of the area surveyed, but is most prevalent where the smallest cod have been found in bottom trawl surveys. The frequency of occurrence of cannibalism recorded in recent years (late 1970s to the present) has usually been low (<I%) in all regions. Much higher values were reported from the Barents Sea in 1949-1960. In the Barents Sea, the frequency of occurrence of cannibalism increased with the abundance of juvenile cod. There was little support for the hypothesis that the frequency of cannibalism increased when the abundance of capelin, the major prey of cod, was low. It can be demonstrated that cod eat large numbers of their young, especially those of ages 0-2, but the mortality rate imposed cannot be calculated without independent estimates of the abundance of young cod. Mortality imposed by cannibalism and competition is estimated without stomach content data for Icelandic cod. This is done by modelling the change in abundance from the pelaglc 0-group stage, as measured by pelagic trawl surveys, to age 3, as estimated by VPA.

Cannibalism and year-class strength in Atlantic cod (Gadus morhua L.) in Arcto-boreal ecosystems (Barents Sea, Iceland, and eastern Newfoundland)

Some instances of cod and haddock behaviour and concentrations in the Newfoundland and Labrador areas in relation to food

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Some instances of cod and haddock behaviour and concentrations in the Newfoundland and Labrador areas in relation to food