Variations in the contribution of transport to changes in planktonic animal abundance: a study of the flux of fish larvae in Conception Bay, Newfoundland

Abstract

We investigated the magnitude of short-term variations in the flux of several species of fish larvae in Conception Bay, Newfoundland, determined the contribution of these fluxes to changes in the abundance of these species, and assessed the factors that may be causing the variations in these fluxes. The net effect of transport ranged from a daily influx of 5.8% to a daily efflux of 6.2% from the population. Short-term variations in transport were associated with variations in wind stress. The contribution of transport to variations in population abundance ranged from 12 to more than 75% of the average daily rate of change in numbers for the different species. The contribution of physical processes to estimated mortality rates is variable and can have a significant impact on the investigation and interpretation of potential biological processes (e.g., predation and starvation) influencing the survival of larval fish. We predicted that the average correction to the instantaneous mortality rate due to advection and diffusion should be proportional to A−1/2, where A is the area of system under study. This highlights the susceptibility of small systems to estimation error of mortality rates caused by short-term variations in physical forcing.

Passive and active behavioural contributions to patchiness and spatial pattern during the early life history of marine fishes

“…By contrast, shifts in the size frequency distribution of larvae from demersal eggs such as sandlance show an increase in size from spring to summer, a pattern that is consistent with the retention of growing individuals. Although the size differences observed are suggestive of slow growth rates (Pepin et al 1995), surface waters in Placentia Bay are relatively cold at the time of year when sandlance are most numerous, and slow growth rates would therefore be expected. The contrasting findings for cod and sandlance are consistent with the hypothesis that larvae from demersal spawners are more highly developed at hatch than those from demersal spawners (Thresher 1984), and are therefore capable of maintaining their association with the coastal environment (P. V. R. Snelgrove unpubl.…”

“…Second, when swim speeds are compared to average flows representative of Placentia Bay (Schillinger et al 2000) and other Newfoundland embayments (e.g. Pepin et al 1995), swim speeds of larvae hatched from demersal eggs are well within the ranges of typical ambient flow. The same can be said only for the most advanced and largest larvae from pelagic spawners.…”

“…The majority of marine organisms possess a pelagic phase, and distribution during this stage is believed to be a complex interaction of hydrography (e.g. Pepin et al 1995) and behaviour (e.g. Butman et al 1988).…”

Modelling Temperature, Currents and Stratification in Placentia Bay

“…Compared with offshore habitats, coastal spawning may lead to increased survival because of higher growth rates (Frank and Leggett, 1982;Taggart and Leggett, 1987;Pepin et al, 1995). As a result, following the right site hypothesis proposed by deYoung and Rose (1993) for Atlantic cod, Placentia Bay is one of the few areas in Atlantic Canada where spawner biomass and, therefore, egg and larval supplies are reasonably healthy.…”

“…In the coastal embayments of Newfoundland, such as Placentia Bay, currents are less influenced by the cold Labrador Current and more by local processes, such as wind forcing which can contribute to both transport and vertical mixing, thereby influencing the abundance and distribution of larval fish (Pepin et al, 1995).…”