Читать книгу Fish and Fisheries in Estuaries - Группа авторов - Страница 73

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.