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ABSTRACT. We investigated the influence of variations in the size of prey (Mallotus villosus) and a vertebrate predator (Gasterosteus aculeatus) on larval fish mortality rates during the period of yolk absorption using mid-size mesocosms (2.7 m". Increasing predator size increased mortality rates of capelin larvae. Variations in larval capelin size resulted in 2 distinct patterns. Between experimental trials, greater mean size of larval capelin in the mesocosm reduced mortality due to predation. Within an experimental mesocosm, larger larvae suffered higher mortality than smaller individuals. Contrasting patterns of size-dependent vulnerability to predation reflect the hierarchy of processes that determine the probability that a larval fish will be preyed upon. The broad scale response of the predator was determined by the mean relative sizes of prey and predator which govern the average probabilities of encounter, attack and capture. Within the search ambit of a predator (e.g area or volume searched within a complete diurnal foraging cycle) active prey selection for larger prey due to either greater visibility or higher energy reward was an important factor. A comparison of our results with estimated predation rates by the jellyfish Aurelia aurita indicates that at a similar size a gelatinous zooplankter consumes fewer larvae than a stickleback and is a less efficient predator as measured by the energy ingested relative to energy demands. For both vertebrate and invertebrate predators, the ratio of prey to predator lengths was a strong predictor of the daily mortality rate due to predation. Relative prey-predator sizes may provide a useful perspective to assess changes in larval fish vulnerability as they grow through a predator field. INTRODUCTIONCumulative survival during the early life history of fish is influenced by both growth and mortality rates (Houde 1987, Beyer 1989, Pepin 1991 1982, Christensen 1983, Alheit 1986, Folkvord & Hunter 1986, Pepin et al. 1987, Fuiman & Gamble 1988, Fuiman 1989 predators can feed heavily on the early life history stages of fish. Second, predation rates on fish larvae by invertebrate carnivores tend to decrease as larvae increase in size (Theilacker & Lasker 1974, Bailey 1984, Bailey & Batty 1984, Purcell et al. 1987) whereas the predation rates on fish larvae by vertebrate predators first increase as larvae increase in size (Folkvord & Hunter 1986, Pepin et al. 1987, Fuiman 1989, Fuiman & Gamble 1989) but eventually decrease as the larvae become more capable of avoiding attacks (Fuiman 1989). The pattern of vulnerability of larval fish to different predator types is O Inter-Research/Printed in Germany

Between 1982 and 1991, an annual survey of stage I egg production of Atlantic mackerel (Scomber scombrus) was conducted in June/early July in the southern Gulf of St Lawrence. We investigated the relationship between interannual variability in biomass of zooplankton, determined from the archived survey plankton samples, and mackerel recruitment, estimated from the proportion of three‐year‐olds in the catch of the commercial fishery. Zooplankton biomass varied by a factor of 2.5, primarily owing to fluctuations in the >1000 μm size fraction. The index of mackerel recruitment fluctuated by a factor of ≈20 and was positively related (linear regression: P < 0.05; n = 10) to variations in the zooplankton biomass. Both mackerel recruitment and zooplankton biomass were negatively related (linear regression: P < 0.05) to RIVSUM, a measure of freshwater discharge from the St Lawrence River system and an index of variability in the region's climate. Three hypotheses are put forward to explain these observations: (1) there is a strong link between interannual variation in abundance of copepod females, which produce prey for mackerel larvae, and larval survival; the exceptional recruitment and subsequent year class resulted from an exceptional production of Calanus finmarchicus nauplii; (2) years of high zooplankton biomass provide better feeding conditions and consequently higher survival of mackerel juveniles; and (3) mackerel recruitment and zooplankton biomass are independently under the control of an underlying physical process, without strong trophic linkage. The first hypothesis is supported by a study of copepod species composition and female abundance conducted for four of the survey years. At the present time, none of these hypotheses can be ruled out.

In June 1989, the water column along a transect in the north‐central Gulf of St Lawrence was thermally stratified (10–14oC at the surface; 0‐loC at 30 m). In the surface layer, nitrate and chlorophyll concentrations were very low; the mean concentration of chlorophyll a > 5 μm in the subsurface maximum was 0.26 μg1‐1. Autotrophic and (presumably) heterotrophic flagellates and dinoflagellates were the most abundant micro‐plankton. In this system, redfish (Sebastes spp.) larvae and the planktonic copepod Calanus finmarchicus overwhelmingly dominated the ichthyoplankton and zoo‐plankton, respectively. Redfish larvae, Calanus females and Calanus eggs were most abundant in the surface layer (0–25 m) day and night. Daily specific egg production rates of Calanus, calculated from shipboard incubations of females, approached the predicted maximal level for this species at the ambient temperature of the surface layer, indicating no or little food limitation. The redfish larvae were feeding almost exclusively on the Calanus eggs and were found in greatest abundance along the transect where Calanus egg production rates (eggs m‐2 day‐1), calculated from the product of the specific egg production rate and female concentration, were highest. The mono‐specificity of the larval redfish diet and the codom‐