Spawning pattern variability and its effect on retention, larval growth and recruitment in Georges Bank cod and haddock

The MARMAP 11-yr time series (1977 to 1987) of cod and haddock egg/larval distributions was used as a basis for examining the consequences of spawning between eastern and western Georges Bank. For each year, from 1978 to 1987, weekly mean flow fields were based on linear interpolation of seasonal velocity fields with wind and upstream boundary-forcing anomalies. Particles were released from the Northeast Peak (NEP) and western Georges Bank (WGB) spawning areas to assess the relative number of larvae retained within the 70 in isobath after 8 wk. A scaled aggregate Georges Bank (AGB) retention index was also created by weighting the relative number of eggs spawned on the eastern and western halves. While there was considerable interannual variability, mean retention for particles released from NEP at the surface (0 to 10 in) was low (< 20 %) during winter but increased in the spring to about 50 % by mid-April, declining through May. Mean retention at 30 in, however, was consistently between 30 and 40 %. Mean retention of particles from WGB was higher for surface releases than at depth, and reached 50 % during May. The NEP usually had higher retention than WGB, but the WGB releases could contribute significantly to retention, especially near the surface during the winter period, when there was a loss of particles from NEP due to wind. The 2 yr of high recruitment for cod, 1980 and 1985, occurred during periods of high retention over the spawning season, despite the fact that in 1980 spawning was primarily in the eastern area, while, in 1985, spawning was split between the areas. The large year class of haddock in 1978 had high larval retention and the latest seasonal growth optima in the time series, extending through May.