abundances of exploitable stages earlier than at the age individuals are first fished. Surveys of young‐of‐year juveniles are commonly conducted in estuarine nurseries to provide ‘juvenile indices’ that have utility in forecasting future recruitment (see Figure 3.15). Identifying a life stage that is strongly linked to recruitment variability is critical for forecasting. It is axiomatic that the life stage closest to the recruited stage will be the stage most likely to succeed in forecasting recruitment (e.g. Bradford 1992), but dynamics occurring in that life stage may not have determined the fate of a year class or cohort.
Although forecasting recruitment is accomplished most confidently based on abundances of late‐stage larvae and juveniles (Bradford & Cabana 1997), success in forecasting using those abundances does not necessarily mean that recruitment levels were set in the late‐larval or juvenile stages because dynamics in egg and early‐larval stages could have driven the outcome. This is apparently the case for the pleuronectid Pleuronectes platessa whose offshore survival during egg and early‐larval stages coarsely controls recruitment level. However, fine‐tuning recruitment estimates and acquiring predictive capability are greatly enhanced during the juvenile stage in estuaries and coastal nurseries where regulation occurs (Beverton & Iles 1992, Iles 1994, Van der Veer et al. 1994, 2015). In a paralichthyid Paralichthys lethostigma, meteorological forcing and river discharge at the time of larval ingress were successful in hindcasting recruitment levels of age‐0+ juveniles (Figure 3.20) in North Carolina (USA) estuaries (Taylor et al. 2010). In this example, relative abundances of age‐1–3 individuals could be predicted based upon age‐0+ abundances that were modelled from larval ingress and environmental forcing variables. The inclusion of river discharge in the P. lethostigma model may be especially appropriate because age‐0+ individuals settle and concentrate in the fresher waters of estuaries (Lowe et al. 2011).
In the Baltic Sea, progress in predicting recruitments of clupeid fishes has evolved by considering climate, hydrographic and other environmental factors (e.g. temperatures, ice cover, larval prey abundance) in modelling the recruitment process. For the Gulf of Riga Clupea harengus, year‐class abundance was predicted from the mean water temperature of the 0–20 m depth layer in May and the biomass of a key larval prey, the copepod Eurytemora affinis (ICES 2009). Also, research on environmental variables was conducted to understand and predict recruitment of Sprattus sprattus, focusing on important factors such as spring temperatures, ice coverage and the North Atlantic Oscillation climate variable (MacKenzie & Köster 2004, MacKenzie et al. 2008). While recruitment levels of clupeid species and stocks in the Baltic may be influenced by numerous factors, climate indicators, especially temperature, and spawning stock biomass (for C. harengus) were important variables for predicting recruitment (Margonski et al. 2010).
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,
