on the short‐lived, estuarine atherinid Menidia menidia, Conover & Munch (2002) and Conover & Baumann (2009) found strong inferential evidence suggesting that under heavy, age‐ or size‐selective exploitation, a population's reproductive potential could evolve rapidly. For M. menidia experiencing age‐ or size‐selective harvesting of larger, faster‐growing individuals, progeny matured at younger ages and smaller sizes than in unexploited populations. There had been a genetic shift towards production of slower‐growing, earlier‐maturing M. menidia in only four generations. Size‐selective fishing is common in coastal and estuarine fisheries, and it is probable that heavily exploited species in estuaries are more prone to selective harvesting than offshore species. For example, size‐selective fishing has been shown to favour fast‐growing individuals that mature at smaller sizes and younger ages in the anadromous osmerid Osmerus eperlanus from the Gulf of Riga, Baltic Sea (Arula et al. 2017) and Osmerus mordax in the northeastern USA (Chase et al. 2019).
The pleuronectid Pleuronectes platessa has experienced declines in size at maturity and shifts in growth rate and age structure in the southern North Sea in the past century that are attributed to changing fishing mortality rates. Rijnsdorp (1993) inferred that these changes were in part genetic, but not independent of concurrent changes in productivity of juvenile coastal and estuarine habitats. He also found that, while growth dynamics changed, the reproductive investment by females did not (i.e. relative ovary weights of females remained constant), although there was a shift in recent decades towards larger but fewer eggs in ovaries (Rijnsdorp 1993). In laboratory experiments on P. platessa, Kennedy et al. (2007) concluded that larger females produce bigger eggs and larvae with greater yolk volumes that grew faster than larvae from eggs of smaller females. Maternal effects on spawning also were demonstrated in laboratory experiments on the estuarine pleuronectid Pseudopleuronectes americanus (Buckley et al. 1991). In a protracted spawning season, large, early‐spawning females produced bigger eggs than those produced by small, late‐spawning females, implying that conservation of large females in P. americanus will maintain large egg sizes, overall fecundity and presumably increased survival potential of early‐life stages. Overall, there is compelling evidence that selective forces (primarily fishing) can modify age structure and precipitate changes in reproductive and recruitment potential of estuary‐dependent fishes such as P. platessa and P. americanus.
In the anadromous moronid Morone saxatilis from the east coast of North America, effects of changes in age structure of the adult stock and its effects on reproductive success and recruitment potential are well documented (Secor 2000). Overfishing led to stock depletion, a severe reduction in abundance of mature, older age classes and recruitment failures (Richards & Rago 1999). Secor (2000) demonstrated that recovery and conservation of older age classes promoted diversity in the timing of spawning amongst age classes in Chesapeake Bay, which was important for sustained reproductive success. Older and larger females produce larger eggs and larvae (Zastrow et al. 1989). In laboratory experiments, larger larvae maintained their size advantage from hatching throughout a 25‐day experimental period (Monteleone & Houde 1990), conferring a potential survival advantage and arguing in support of Secor's (2000) recommendation to conserve age diversity of spawning adults.
In anadromous Pacific salmonids, heavy exploitation over decades of selective fishing on larger and faster‐growing individuals has affected sizes and age structure of spawners such that adults are smaller now than historically (Ricker 1981). The reduction in spawner size was especially notable for Oncorhynchus tshawytscha whose body weights declined to less than 80% of historical weights. The effect was also substantial in O. kisutch, O. gorbuscha and O. keta (Ricker 1981). The implications are that reduced fecundities, smaller egg sizes and reduced egg quality can lower fitness and recruitment potential. Recently, the decline in spawner weights of Pacific salmonids has stabilised for O. gorbuscha and O. keta and reversed in O. tshawytscha and O. kisutch (Jeffrey et al. 2017), indicating that recovery of spawning potential from effects of excessive exploitation is possible through appropriate management.
3.4.2 Scales and patterns of variability in reproductive success
Annual variability in recruitments of marine and estuarine fishes may differ by an order of magnitude or more (Cushing 1981, Rothschild 1986, Houde 2016). Inter‐annual variability and long‐term trends in recruitment document the level of reproductive success in marine and estuarine fish stocks. The scales, patterns and temporal trends of recruitment variability in estuary‐dependent and ‐associated fishes are discussed in the following sections.
3.4.2.1 Recruitment levels and variability
Inter‐annual variability in recruitment of estuary‐dependent and ‐associated fishes is often cued to weather and climate patterns that differ inter‐annually or amongst climate regimes (Wood & Austin 2009). Recruitments sometimes are autocorrelated, reflecting successive years of similar weather patterns that contribute to either low or high reproductive and recruitment success, or to time periods of alternating high and low recruitment (Fogarty 1993), often labelled as regime shifts. Decadal trends in recruitments of estuary‐dependent and ‐associated fishes are sometimes concordant with climate shifts and decadal (or longer) climate regimes (Wood 2000, Hare & Able 2007, Wood & Austin 2009, Hare et al. 2010, Nye et al. 2014). Long‐term shifts in patterns in recruitment and reproductive success, particularly declines, may be attributed to deterioration of estuarine habitat and water quality resulting from human activities (Houde et al. 2014).
Variable patterns in levels of reproductive success have been documented for anadromous fishes that spawn in estuaries on the east coast of North America and for marine and estuary‐dependent fishes that spawn offshore in the western North Atlantic (Hare et al. 2010, Nye et al. 2014). In Chesapeake Bay, recruitment of anadromous species that enter the bay to spawn is relatively successful in years that are cool, experience high precipitation and above‐average freshwater discharge during the spawning and nursery seasons. In contrast, offshore spawners, whose young use Chesapeake Bay as a nursery, experience successful recruitments in years when regional weather is warm and dry (Wood 2000, Kimmel et al. 2009, Wood & Austin 2009). There are exceptions, however, for example, glass eels of the catadromous Anguilla rostrata experience greater ingress success to estuaries on the US east coast in years with high precipitation (Sullivan et al. 2006). High precipitation and associated river flows also are associated with higher recruitments of A. anguilla glass eels to European estuaries, although the highest flows may counteract the efficiency of selective tidal stream transport (STST) in facilitating up‐estuary migrations (Harrison et al. 2014).
In the Baltic Sea, spawning stock biomasses of clupeids (Clupea harengus and Sprattus sprattus) and the gadid Gadus morhua are only weakly correlated with recruitments (ICES 2018). It is hypothesised that the observed variability in recruitment is largely due to variability in high mortality rates during early life (Ojaveer 1988). Recruitment of the Gulf of Riga stock of C. harengus is influenced by a suite of factors, including spawning stock biomass, the winter Baltic Sea Index prior to spawning (Lehmann et al. 2002) and sea surface temperature during winter after fall spawning (Ojaveer 1988). Recruitment level of the Gulf of Riga spring‐spawning C. harengus has varied more than tenfold in the 1957–2011 period (Arula et al. 2014). In contrast, recruitment variability in the spring‐spawning, Central Baltic stock of C. harengus is related to summer (August) sea surface temperature and its apparent effect on survival of larvae and juveniles. Recruitment level of Sprattus sprattus is positively related to summer (July–August) temperature and to spawning stock biomass when SSB is <200 000 tonnes (Margonski et al. 2010, ICES 2018). Recruitment of the Eastern Baltic gadid G. morhua is significantly related to spawning stock biomass, the winter North Atlantic Oscillation index and the varying reproductive volume of the Gotland Basin (ICES 2018). Assessment models for Baltic fishes that include extrinsic factors,
