In the temperate Chesapeake Bay, recruitment of the abundant, unfished engraulid Anchoa mitchilli occurs at <4 months posthatch and varies at least ninefold amongst years (Jung & Houde 2004a) while young‐of‐the‐year, juvenile abundance of the moronid Morone saxatilis (Figure 3.15) varies >30‐fold (Martino & Houde 2012). In the case of M. saxatilis, whose lifespan may exceed 35 years, recruitment to the fishery does not occur until age 3 and, upon entry to the fishery, recruited year classes on the east coast of North America vary only threefold (NEFSC 2019). On the west coast of North America, where introduced M. saxatilis is established in the San Francisco Bay Estuary, its year classes vary >25‐fold as age‐0+ juveniles but abundances are less variable (approximately threefold) at age of recruitment to the fishery (Kimmerer et al. 2000, 2001). The reduction in variability in cohort abundances from age‐0+ to age‐3 is attributed to density‐dependent regulation during the juvenile stage.
3.4.2.2 Adult stock and recruitment
In managed fisheries, developing stock‐recruitment (S‐R) models is an established element of the assessment process aimed at determining management reference points. For example, reference points for maximum sustainable yield (MSY), fishing mortality corresponding to MSY (FMSY) and the biomass corresponding to MSY (BMSY) depend on estimates of recruitment derived from S‐R models (Maunder 2012). Unidentified variability in S‐R relationships of estuary‐dependent fishes, as in other marine fishes, is attributable to inaccuracies in estimating stock and recruit abundances and to the dominant role of environmental factors that generate recruitment variability (Cushing 1975, Rothschild 1986, Hilborn & Walters 1992). Most of the variability is generated by environmental factors acting on early‐life stages (e.g. Fogarty 1993), but substantial variability, especially at low adult stock abundance, is associated with the level of spawning stock biomass and stock fecundity (Myers 2001).
Figure 3.15 Morone saxatilis young‐of‐the‐year (YOY) from Chesapeake Bay (USA) seine net survey data showing geometric mean abundances (Catch Per Unit Effort, CPUE). Note the ~30‐fold differences amongst years which are reduced to three‐ to fivefold variability by age‐3
(from Maryland Department of Natural Resources 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
