https://dnr.maryland.gov/fisheries/Pages/striped‐bass/juvenile‐index.aspx).
In broad analyses of stock and recruitment relationships across taxa of fishes, including many estuary‐resident or estuary‐dependent species, broad conclusions indicated that high recruitment usually occurs when spawning stock biomass is above the long‐term median, and low recruitment often is associated with low spawning‐stock abundance (Myers & Barrowman 1996, Myers 2001, Cury et al. 2014). For taxa with estuarine or estuary‐dependent stocks (e.g. Clupeidae, Salmonidae, Pleuronectiformes), relationships were variable. Clupeidae and Salmonidae tended to show strong, positive relationships between recruitment and level of spawning stock, while Pleuronectiformes had weak, non‐significant relationships that could be attributed to strong density‐dependent regulation of survival in juveniles on estuarine nursery grounds (Beverton & Iles 1992, Iles 1994, Myers & Barrowman 1996).
Several S‐R models have been proposed and applied to marine and estuarine fishes. Iles (1994) reviewed the models with emphasis on pleuronectiforms, particularly estuary‐dependent species. The two models most used (Figure 3.16a) are those developed by Ricker (1954, 1975) and Beverton & Holt (1957):
where R is recruitment, P is adult stock, α is a density‐independent coefficient and β is a density‐dependent coefficient.
Regulation of recruitment levels, as observed in S‐R relationships, is established through density‐dependent recruitment, in which compensatory mortality and growth rates operate during early life. For many species, the compensation may occur in the late‐larval and juvenile stages when prey resources, habitat availability or predation regulate abundance (Iles 1994, Rose et al. 2001). The ‘concentration hypothesis’ (Beverton 1995) posits, with evidence, that recruited abundances of demersal fishes, exemplified by pleuronectiforms such as Pleuronectes platessa, are regulated, i.e. essentially fine‐tuned by predation on juveniles after settlement in estuarine nurseries. In some estuarine fishes, density‐dependent regulation of age‐0 juveniles occurs overwinter during years of high abundance under stressful winter conditions (Hare & Able 2007, Hurst 2007, Martino & Houde 2012).
Figure 3.16 (a) Generalised stock–recruitment relationships illustrating the Beverton–Holt (asymptotic) and Ricker (domed) relationships. (b) Stock–recruitment relationship for Morone saxatilis on the east coast of North America. A Beverton–Holt stock–recruitment model is fitted to the M. saxatilis data. (b) is from NEFSC (2013, figure B7.9).
An example of a Ricker S‐R relationship for the offshore‐spawning, estuary‐dependent sciaenid Micropogonias undulatus in the western North Atlantic illustrates the domed relationship of recruitment with respect to adult stock biomass (Figure 3.17). This figure also shows how winter temperatures contribute to the high variability in recruitment success (Hare et al. 2010). In the anadromous moronid Morone saxatilis from the Atlantic coast of North America, recruitment is coarsely related to spawning stock, especially at low spawning stock level but becomes increasingly variable when adult stock is high (Figure 3.16b), suggesting a maximum, but highly variable, recruitment level as derived from the fitted Beverton–Holt model (NEFSC 2013). In M. saxatilis, density dependence may operate during the age‐0 juvenile stage (Kimmerer et al. 2000, 2001, Martino & Houde 2012). In some estuarine systems, density‐dependent overwinter mortality occurs and is directed towards smaller juveniles of M. saxatilis in years that had experienced high larval and age‐0 juvenile production (Hurst & Conover 1998, Martino & Houde 2012).
Figure 3.17 Stock and recruitment models for the sciaenid Micropogonias undulatus on the east coast of the USA showing the effects of spawning stock biomass and winter air temperature. (a) Recruitment level with respect to winter air temperature. (b) Ricker stock–recruitment model fitted to levels of recruitment relative to spawning stock biomass for three temperature regimes
(modified from Hare et al. (2010, their figure 1)).
Modelling recruitment and parent stock may be improved by carefully selecting and including environmental variables in the S‐R models, an approach that can be particularly appealing for many estuarine species, as noted above for effects of winter temperature on the sciaenid Micropogonias undulatus (Hare et al. 2010) (Figure 3.17b). For the clupeid Brevoortia patronus in the Gulf of Mexico, Mississippi River discharge is inversely related to recruitment (Govoni 1997). Inclusion of a river flow variable in either a Ricker of Beverton–Holt S‐R model improves the model fit for recruitment of B. patronus sufficiently to provide predictive power (Vaughan et al. 2011). In another example, for the anadromous moronid Morone saxatilis from Chesapeake Bay, including freshwater discharge in spring months as a variable in a Ricker S‐R model increased the model's predictive power (r 2 increased from 0.03 to 0.44) (North & Houde 2003). Furthermore, a modified Ricker S‐R model with both an added environmental variable (salinity) and a stock variable (age diversity of adult spawners) (Figure 3.18) explained a high proportion of the variability in observed recruitments of age‐0+ juvenile M. saxatilis (Houde 2008).
For the anadromous alosine Alosa sapidissima in the Connecticut River (USA), a Ricker model fitted to spawner abundance explained little of the observed recruitment variability (Crecco & Savoy 1984, Crecco et al. 1986). Including a freshwater‐discharge variable in the S‐R model improved the modelled result (r 2 increased from 0.02 to 0.26). This relationship, while still far from a reliable predictor of recruitment, convincingly indicated that freshwater discharge (in this case a negative factor) may control recruitment and, in many years, may be more important than abundance of adult spawners.
Figure 3.18 Modified Ricker stock–recruitment model for Morone saxatilis, Chesapeake Bay. Juvenile (age‐0+) abundance index, i.e. Recruitment (R), Adult stock biomass (P), Salinity (Sal), and spawner age diversity (H′). H′ is the Shannon–Wiener Diversity Index applied to the abundances‐at‐age of adult spawners in each year
(from Houde (2008, his figure 6)).
Recruitment levels of an unfished species, the short‐lived engraulid Anchoa mitchilli, in Chesapeake Bay were described by a modified Ricker S‐R model with ΔL (a latitudinal variable that defines the centroid