Dispersal Modeling of Fish Early Life Stages: Sensitivity with Application to Atlantic Cod in the Western Gulf of Maine

As an initial step in establishing mechanistic relationships between environmental variability and recruitment in Atlantic cod Gadhus morhua along the coast of the western Gulf of Maine, we assessed transport success of larvae from major spawning grounds to nursery areas with particle tracking using the unstructured grid model FVCOM (finite volume coastal ocean model). In coastal areas, dispersal of early planktonic life stages of fish and invertebrate species is highly dependent on the regional dynamics and its variability, which has to be captured by our models. With state-of-the-art forcing for the year 1995, we evaluate the sensitivity of particle dispersal to the timing and location of spawning, the spatial and temporal resolution of the model, and the vertical mixing scheme. A 3 d frequency for the release of particles is necessary to capture the effect of the circulation variability into an averaged dispersal pattern of the spawning season. The analysis of sensitivity to model setup showed that a higher resolution mesh, tidal forcing, and current variability do not change the general pattern of connectivity, but do tend to increase within-site retention. Our results indicate strong downstream connectivity among spawning grounds and higher chances for successful transport from spawning areas closer to the coast. The model run for January egg release indicates 1 to 19 % within-spawning ground retention of initial particles, which may be sufficient to sustain local populations. A systematic sensitivity analysis still needs to be conducted to determine the minimum mesh and forcing resolution that adequately resolves the complex dynamics of the western Gulf of Maine. Other sources of variability, i.e. large-scale upstream forcing and the biological environment, also need to be considered in future studies of the interannual variability in transport and survival of the early life stages of cod.

Use of Otolith Strontium:Calcium Ratios for Hindcasting Larval Cod Gadus Morhua Distributions Relative to Water Masses on Georges Bank

The concentration ratios of strontium to calcium in laboratory-reared larval cod otoliths are shown to be related to the water temperature (T) at the time of otolith precipitation. This relationship is curvilinear, and is best described by a simple exponential equation of the form (Sr/Ca x 1000 = a exp(-T/b). We show that when Sr/Ca elemental analyses are related to the daily growth increments in the larval otoliths, relative temperature histories of individual field-caught larvae can be reconstructed from the egg stage to the time of capture. We present preliminary examples of how such reconstructed temperature histories of Atlantic cod Gadus morhua larvae, collected on Georges Bank during April and May 1993, may be interpreted in relation to the broad-scale larval distributions and the hydrography of the Bank.

Estimates of in situ Larval Development Time for the Lobster, Homarus Americanus

Larval development time is a critical factor in assessing the potential for larval transport, mortality. and subsequently, the connectivity of marine populations through larval exchange. Most estimates of larval duration are based on laboratory studies and may not reflect development times in nature. For larvae of the American lobster (Homarus americanus), temperature-dependent development times have been established in previous laboratory studies. Here, we used the timing of seasonal abundance curves for newly hatched larvae (stage 1) and the final plankonic instar (postlarva), coupled with a model of temperature-dependent development to assess development time in the field. We were unable to reproduce the timing of the seasonal abundance curves using laboratory development rates in our model. Our results suggest that larval development in situ may be twice as fast as reported laboratory rates. This will result in reduced estimates of larval transport potential, and increased estimates of instantaneous mortality rate and production.