alt="Schematic illustration of modelled hindcasts of Paralichthys lethostigma recruitments."/>
Figure 3.20 Modelled hindcasts of Paralichthys lethostigma recruitments (JAI = Juvenile Abundance Index) in North Carolina Sounds (USA) for a series of years. Winds and river discharges were the predicting variables. Points represent observed values; line represents model predictions
(data from 1987 to 2002 were used to parameterise the model. From Taylor et al. (2010, their figure 9)).
3.4.3 Recruitment: an integrated, evolved process
It is generally recognised that no single process or mechanism is responsible for recruitment variability in estuary‐dependent and ‐associated fishes. Decades ago, Cushing (1975) referred to reproduction and recruitment in fishes as a ‘single process’, dependent on evolved dynamics in multiple life stages that ensures replenishment and maintenance of stocks. Recruitment success can depend on variability in survival during all life stages. Numerous factors may act in concert or in an integrated fashion over the entire egg to juvenile period, and the abundance and condition of adults may also affect recruitment outcomes (e.g. Rothschild 2000, Marshall 2016).
The juvenile stage is increasingly recognised as key to replenishment success in many fishes (Bradford & Cabana 1997). In estuary‐dependent and ‐associated fishes, the juvenile stage features transitions, resulting from ontogeny but also associated with occupation of new habitats. While most mortality in the egg and larval stages may be density independent and attributed to environmental factors, a substantial density‐dependent component often emerges after metamorphosis or settlement. Density‐dependent mortality arises from resource limitation that potentially occurs when growth of abundant, newly settled fishes is retarded, rendering the settlers vulnerable to size‐selective predators during the juvenile stage (Van der Veer 1986, Houde 1987, Beverton & Iles 1992, Myers & Cadigan 1993, Rose et al. 2001). A classic example is that for the newly settled pleuronectid Pleuronectes platessa and invertebrate predators in the Wadden Sea (Beverton & Iles 1992, Iles 1994). In another example, density‐dependent mortality in age‐0+ juveniles in years of high larval production is an important regulator of recruitment of the moronid Morone saxatilis in San Francisco and Chesapeake Bays (Kimmerer et al. 2000, Martino & Houde 2012). Similarly, Blaber (1973) recorded higher juvenile mortality rates of the sparid Rhabdosargus holubi in the West Kleinemonde (South Africa) Estuary following a good year of larval ingress compared to a poor year.
Processes controlling recruitment in fishes have evolved to promote reproductive resilience and to ensure a degree of stability over time (Lowerre‐Barbieri et al. 2016). The integration of processes across life stages is particularly important for successful recruitment in estuarine fishes that have complex life histories. Connectivity pathways in migrations that link life stages have evolved for many estuarine and anadromous fishes that resemble, at least in concept, the migration triangles proposed by Harden‐Jones (1968) and discussed by Cushing (1975) and Secor (2002). In this conceptual view, adults migrate to spawn in areas that are trophodynamically reliable for larval feeding and which have hydrodynamics conducive for retention or transport of larvae to juvenile nurseries. In some species the triangle is closed by the migration of recruits (juveniles) to areas occupied by the recruited stock. The properties of enrichment, concentration and retention proposed by Bakun (1996) in his ocean triad hypothesis as critical to recruitment success were developed with upwelling ocean ecosystems in mind, but these properties are also important in estuaries to ensure successful reproduction, production of young fish, and eventual recruitment.
3.5 Threats to reproduction and recruitment in estuaries
Threats to reproductive and recruitment success of estuary‐dependent and ‐associated fishes include natural and anthropogenic stressors. We briefly review these threats, primarily addressing human threats. We refer readers to chapters that address other aspects of threats to estuary‐associated fishes and fisheries; for example, Fishes and Estuarine Environmental Health (Cabral et al. 2022), Global and Climate Change Trends (Gillanders et al. 2022) and Estuarine Degradation and Rehabilitation (Lepage et al. 2022) for additional information.
As human impacts on estuaries have increased worldwide, multiple stressors associated with anthropogenic activities and inputs have lowered the productive and reproductive potential of fish populations in estuaries (Breitburg et al. 2015, Toft et al. 2018). The interactions amongst stressors and cumulative effects of multiple stressors may reduce productivity of spawning and nursery habitats (Breitburg et al. 2009, Breitburg et al. 2015, Elliott et al. 2019). The most observable short‐term threats in estuaries are overfishing and fish kills, which can remove large numbers of juveniles and adults from populations and thus eliminate their reproductive potential. Fish kills in estuaries are reported (Biernacki 1979, Burkholder et al. 1995, Whitfield 1995, Whitfield & Cowley 2018) from both natural and anthropogenic causes, but sub‐lethal impacts often are not appreciated (Rose 2000, O'Connell et al. 2004). Below, we briefly describe threats to reproduction and recruitment of estuary‐associated fishes. These threats are largely attributable to human activities (e.g. overfishing; habitat destruction; impoundments and freshwater‐flow regulation; power plants; deteriorating water quality with associated increases in contaminants; eutrophication; urbanisation; climate change and associated degradation and manmade and natural catastrophes). These occur disproportionately in estuaries and their catchments relative to other marine ecosystems, adding to concerns regarding threats to success of reproduction and recruitment in estuary‐associated fishes.
3.5.1 Excessive fishing: depletion of adults and by‐catch of juveniles
Heavy exploitation and excessive fishing, including by‐catch, have led to depletion of some estuary‐dependent and ‐associated fish stocks (Blaber et al. 2000). The cases for salmonids are well known (Levings 2016) in which overfishing is a major factor leading to declining recruitment. In a well‐documented case for a moronid, Morone saxatilis was overfished on the east coast of the USA, leading to failed recruitment and declared fishing moratoria (Richards & Rago 1999). Declines in alosine fishes in the northern hemisphere are in part due to overfishing, including unintended by‐catch, combined with habitat degradation (Limburg & Waldman 2009). These same factors are responsible for declines in recruitment of anguillids (Tesch 2003) and acipenserids (Van Winkle et al. 2002) in the Northern Hemisphere, as well as sciaenids (Cowley et al. 2008) and sparids (Bennett et al. 2017) in the Southern Hemisphere. Unintended by‐catch of adult and juvenile fishes is problematic in many fisheries (Davis 2002) and may be a threat to reproduction and recruitment success in estuary‐associated fishes. A notable example is that for by‐catch of juveniles of the sciaenid Micropogonias undulatus in shrimp trawl fisheries in which the large by‐catches may have different impacts on regional recruitment levels in the southeastern USA and Gulf of Mexico (Diamond et al. 1999, 2000).
3.5.2 Habitat destruction and degradation
Habitat destruction in estuaries began with human settlement and conversion of many marshes into agricultural lands or modified by dredging and filling (Gedan et al. 2009, Gedan and Silliman 2009). Another obvious and ongoing form of habitat destruction results from the cumulative effects of urbanisation (Wolanski et al. 2019).
In Chesapeake Bay, increasing sediment loads and higher turbidity have led to lower seagrass abundance and productivity (Kemp et al. 2005), reducing seagrass habitats that are important for juvenile fish production. Somewhat similarly, deforestation in the Columbia River watershed has increased sedimentation and decreased stream shading and thus negatively affects Pacific salmon habitats that serve as spawning areas and nurseries (Kelly et al. 1999). One subtle but frequent form of habitat degradation in estuaries is the elimination and fragmentation of nearshore, shallow‐water habitats through dredging, filling and hardening of shorelines that often