Regional growth estimates of Atlantic cod, Gadus morhua: Applications of the maximum likelihood GROTAG model to tagging data in the Gulf of Maine (USA/Canada) region

The maximum likelihood GROTAG model was applied to mark-recapture data collected by the region-wide, collaborative ‘Northeast Regional Cod Tagging Program’ to provide new, supplementary growth estimates for Atlantic cod, Gadus morhua, in the Gulf of Maine (USA/Canada) region. The dataset (114,467 releases and 6334 recaptures) were filtered for quality, resulting in 3675 growth records which were categorized into stock management areas (5Y, 5Z and 4X) and fine-scale general areas (Inshore Gulf of Maine, Georges Bank, Cape Cod and the Bay of Fundy). Growth seasonality was detected and region-wide, growth peaked during July. The overall regional von Bertalanffy estimate for cod was L∞ = 135.3 cm and K = 0.17, however statistically significant variation was found between management areas and between general areas throughout the study region. The fastest growth (K = 0.31) and smallest asymptotic size (L∞ = 105.7) were found for 5Z cod, while cod in 5Y showed slower growth (K = 0.13) but a larger asymptotic size (L∞ = 151.3); estimates for the Canadian management area 4X (L∞ = 117.5, K = 0.22) were most similar to the 5Z estimates. Least squares analysis of length-at-age data yielded comparable results. The Inshore Gulf of Maine estimate (L∞ = 154.5, K = 0.13) was highly consistent with that for management area 5Y; similarly, the Georges Bank estimate (L∞ = 104.1, K = 0.26) supported the 5Z estimate. However, more variability was seen in estimates for the Bay of Fundy (L∞ = 134.2, K = 0.16) and Cape Cod (L∞ = 173.5, K = 0.13). The differing growth rates between regions are thought to be influenced by cod movement patterns, GROTAG methodology and the number and size–structure of fish sampled in each region.