Dispersal modeling of fish early life stages: sensitivity with application to Atlantic cod in the western Gulf of Maine

Abstract

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-theart 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.

Fine-scale spatial and temporal genetic structure of Atlantic cod off the Atlantic coast of the USA

“…This circulation pattern supports connectivity among spawning grounds within the GOM and downstream nurseries within Massachusetts and Cape Cod bays and the Nantucket Shoals (Huret et al 2007). For offshore spawning sites, advection of larvae is likely to occur to sites farther offshore (Huret et al 2007). Larval transport varies both annually and seasonally, however.…”

“…The general cyclonic circulation of the Gulf of Maine Coastal Current does not favor local retention, but rather a southwestward flow to downstream or offshore from juvenile nursery sites. This circulation pattern supports connectivity among spawning grounds within the GOM and downstream nurseries within Massachusetts and Cape Cod bays and the Nantucket Shoals (Huret et al 2007). For offshore spawning sites, advection of larvae is likely to occur to sites farther offshore (Huret et al 2007).…”

“…The outcome of egg and larval transport (dispersal) is a function of spawning location in relation to the coastal currents in the western GOM (Huret et al 2007). The general cyclonic circulation of the Gulf of Maine Coastal Current does not favor local retention, but rather a southwestward flow to downstream or offshore from juvenile nursery sites.…”

“…In the GOM, acoustic tracking of spring-spawning cod in Ipswich Bay revealed that spawning activity is focused at vertically elevated bathymetric features where adults aggregate during the spawning period (Siceloff 2009). Spawning in proximity to these features may promote local retention of larvae within the bay.The outcome of egg and larval transport (dispersal) is a function of spawning location in relation to the coastal currents in the western GOM (Huret et al 2007). The general cyclonic circulation of the Gulf of Maine Coastal Current does not favor local retention, but rather a southwestward flow to downstream or offshore from juvenile nursery sites.…”

“…The anticyclonic, clockwise gyre on Georges Bank (Lough et al 2006) may serve to retain larvae locally and limit their dispersal to either the inshore GOM or southern New England waters. Larvae spawned within the GOM, however, especially on offshore or nearshore sites, may drift with the coastal currents toward Georges Bank (Huret et al 2007), resulting in some weak connectivity among these spawning populations despite limited adult movements. Consistent with this explanation, the spawning aggregations on Stellwagen Bank and Jeffrey's Ledge were the most genetically similar of all stocks to the Georges Bank cod.…”