A general biophysical model of larval cod (Gadus morhua) growth applied to populations on Georges Bank

Abstract

Field-derived growth rates (RNA-DNA based) of cod (Gadus morhua) larvae collected on the southern flank of Georges Bank were higher on average in May 1993 than May 1994, despite the apparent higher abundance of potential prey in 1994. A biophysical modeling study is presented here in which factors are examined that may have led to the difference in population mean growth. A one-dimensional physical model, forced by winds and tides, was used to simulate the vertical structure (of currents, temperature field, and turbulent kinetic energy dissipation rate) following a column of water in a Lagrangian sense at a site on the southern flank of Georges Bank over 5-day periods in late May of 1993 and 1994. The biophysical model and observed zooplankton abundance allowed us to explore the vertical structure and temporal (hourly) evolution of feeding and growth for cod larvae in relation to environmental conditions. Our trophodynamic model is improved over previous versions and now includes the effect of light on larval feeding response, as well as the effect of temperature on larval metabolic costs, ingestion, and digestion. Larval prey profiles, comprising four copepod species, were used from a time series of 1/4-m 2 MOCNESS tows to define the prey field. Data from a collateral time-series of larval gut contents (1-m 2 MOCNESS tows) was used to define maximum ingestion (satiation level) and prey selection. Model outputs provide depth-dependent estimates of growth, prey biomass ingested, larval length, and larval weight. Watercolumn growth-rate profiles were made for four size classes of larvae (5, 6, 7 and 9 mm) under the environmental conditions observed in May 1993 and 1994. A weighted-mean growth rate based on the mean vertical distribution of larvae was estimated for each size class. In all cases, when using all available potential prey, the model-derived 1994 growth rates were higher (by 3-6% day )1 ) than those for 1993. However, simulations in which 7-mm larvae followed the field-derived weighted mean depth over the sampling period, and were limited to their preferred Pseudocalanus prey, resulted in average growth of 12.2% day )1 for 1993 and 9.7% day )1 for 1994. These compared closely to the field growth means of 11.3% day )1 in 1993 and 9.8% day )1 in 1994. Thus, the lower observed growth in May 1994 may have resulted from depth-dependent food limitation and prey-selectivity coupled with the greater metabolic costs induced by the higher temperature that year.

Contribution of individual-based coupled physical–biological models to understanding recruitment in marine fish populations

“…ICPBMs induce variability in growth from spatially explicit differences induced by temperature (Heath & Gallego 1998, Brickman & Frank 2000 or the prey field exploited (Werner et al 1995, Lough et al 2005. However, the majority of ICPBMs have assumed that this is the only source of individual variability.…”

“…For example, Bartsch & Combs (2004) used satellite-derived sea-surface temperature and chlorophyll fields to model the abundance of Calanus and Acartia, principal prey for larval mackerel Scomber scrombrus. Other researchers based the prey field on data interpolated from field collections (Lough et al 2005).…”

“…All feeding routines included prey selection to some extent. Some models included a highly detailed and mechanistic description of feeding (Megrey & Hinckley 2001, Fiksen & MacKenzie 2002, Hinrichsen et al 2003b, Lough et al 2005. For example, Fiksen & MacKenzie (2002) developed a highly detailed model of the feeding process that modeled prey encounter and capture as explicit functions of light, turbulence, prey size and vagility, and of larval feeding behavior.…”

“…The other principal 3-dimensional numerical simulation models were ones that used a finite element (FEM) algorithm rather than the finite difference scheme. These FEM models share a common heritage (Lynch et al 1996), and have been implemented for the Northwest Atlantic shelf (Werner et al 1994, Lough et al 2005) and the mid-Atlantic Bight , Stegmann et al 1999. A small number of ICPBMs that did not use numerical 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 Year Number of papers simulation models as their foundation were identified in the review.…”

“…ICPBMs now play a central role in the majority of large-scale fisheries oceanography programs around the world, e.g. FOCI in Alaska (Megrey et al 2002), various national GLOBEC programs (Hinrichsen et al 2002, Lough et al 2005) and the IDYLE program off South Africa . Therefore, it is an appropriate time to assess the contribution of ICPBMs to knowledge of recruitment in fish.…”