Detecting an environmental gradient in maturity, spawning rates, and fecundity of inshore winter flounder stocks: does thermal habitat create spatial heterogeneity of life history parameters within stock boundaries?

Many fish species are widely distributed so their thermal habitat varies from one end of their range to another. Winter flounder (Fig. 1), an important flatfish of the northeast U.S., spawns in coastal waters from Canada to the U.S. middle Atlantic seaboard, as well as offshore, on Georges Bank. Inshore temperatures within its range differ by more than 15oF during the summer, and flounder growth rates vary significantly from north to south. Partly because of these differences, flounder in the U.S. are treated as three stocks: one inshore stock north of Cape Cod (Gulf of Maine), another inshore stock south of Cape Cod (southern New England/middle Atlantic), and the offshore stock on Georges Bank. Age at maturity, which is a primary data input into stock assessments, is also known to differ between these two inshore stocks.

In this study we asked if age at maturity varied within each of these broad stock boundaries. In particular, we were interested in whether the variation in maturity schedules within a stock was gradual or exhibited sharp changes near prominent capes, such as Cape Ann or Cape Elizabeth, in the Gulf of Maine.

We collected female flounder by several surveys: a federal survey (NOAA-NMFS-NEFSC), and similar groundfish surveys operated by cooperating state partners (Connecticut, Maine, Massachusetts, and New Jersey). Maturity was evaluated using gonad histology and age was determined using otoliths.

The results on maturity are summarized in Winton et al. (2014). A strong gradient of maturity was evident in both inshore stocks (click on map image). Adding temperature to the model improved the model fit, but more variation was explained by including spatial data using generalized additive models. Results of spatially constrained hierarchical agglomerative cluster analysis aligned well with current stock regions but also indicated significant intra-stock variation in maturity, particularly in the Gulf of Maine and in Southern New England; however, inclusion of sub-stock structure cannot be applied to the current stock assessment model.

Work is continuing to assess region-specific (within-stock) values of the number of eggs produced, including evidence of skip spawning.

Winter Flounder

Figure 1. A female winter flounder, ready to spawn. The fish is dissected to display the ovary filled with eggs.

Generalized Additive Model of spatial variation in female winter flounder maturity.

Figure 2. Generalized Additive Model of spatial variation in female winter flounder maturity.