juvenile stages.
Recruitment variability of estuary‐dependent and ‐associated fishes, including causes and trends.The chapter addresses recruitment in its broadest sense, highlighting the delivery and ingress processes that bring offshore larvae to estuaries and the early‐life dynamics of estuary‐associated fishes. Processes of feeding, growth, nutrition and mortality via predation or starvation are described for estuary‐associated fishes. Vital processes and rates are similar to those for other marine fishes. Growth and mortality rates are high and variable. Environmental controls, such as temperature, salinity and precipitation/freshwater discharge are identified as key factors controlling survival and, ultimately, recruitment success in estuary‐associated species.Recruitment is an integrated, linked process in which hydrological, biological and behavioural processes in early‐life stages act to frame recruitment success. The offshore, nearshore, surf zones, estuaries and rivers form a continuum traversed by the early‐life stages of many species. Density‐dependent mortality, often via predation and overwinter losses, is recognised in juveniles of some estuary‐dependent fishes as an important regulator of recruitment level. Weather and climate factors likely play a bigger role in controlling recruitment of estuary‐dependent fishes than of ocean fishes that typically reside in an environment buffered from the variability typical of estuaries.
The adult stage of estuary‐dependent and ‐associated fishes, focusing on contributions by adults to reproductive success and recruitment variability.Abundances of recruited fishes are coarsely dependent on adult abundances and egg production, and on availability of suitable habitats to support spawning adults. As in most marine fishes, variability in recruitment of estuary‐associated species is highly dynamic and, except at low adult stock levels, is poorly related to adult abundance. Modelling the relationship between adult stock and recruitment is an important element of stock assessment by fisheries managers to describe trends and dependencies. Parental (especially maternal) effects on reproductive success and recruitment are notable in some estuary‐associated fishes, for example salmonids, pleuronectids and moronids. Effects of fishing on juvenile and adult abundance/biomass and age structure may not only reduce abundance but also may alter age structure, spawning period phenology and embryo quality of some estuary‐associated fishes. Destruction or reduction in extent and quality of estuary spawning habitat has been a root cause of recruitment failures in some estuary‐associated fishes, e.g. salmonids, acipenserids, osmerids, alosines. Changing climate is also leading to shifts in adult distributions and spawning areas.
Recruitment forecasting; predicting reproductive success of estuarine fishes.Predicting the success of reproduction and levels of recruitment is a goal of fish ecologists and fishery managers. Recruitment in estuary‐associated fishes is difficult to forecast because of complex life cycles, the potential for recruiting young stages to have multiple, ontogeny‐related habitat preferences, and the vulnerability of early‐life stages to weather and changing climate. Still, the accessibility of estuarine habitats to sampling and evaluation of effects of environmental factors on spawning success and recruitment outcomes has led to some notable success in modelling and forecasting recruitment. In many cases, for estuaries under intensive human use, it has been possible to predict trends in recruitment, often negative, that are related to human impact. Predictions of recruitment success can be conducted at different temporal and spatial scales that range from survival of larval‐stage daily cohorts in response to local environmental factors to annual success related to variability in offshore, nearshore and estuarine environments supporting early‐life stages. Extended forecasting of long‐term trends in recruitment is being conducted to account for shifting climate and changes in estuarine habitat.
Threats to reproduction and recruitment of estuary‐dependent and ‐associated fishes.The diversity, health and productivity of estuary‐associated fishes depend on reproductive success. Estuaries, more than other marine ecosystems, are under threat globally from human and natural perturbations. Amongst the prominent threats to reproduction are effects of heavy fishing on stock abundances, impoundments that limit spawning access and connectivity in anadromous fishes, declines in freshwater discharge that are critical to larval‐stage productivity and cueing, urbanisation, climate‐related increases in temperature that may exceed thermal optima or thresholds for young fishes, effects of contaminants and effects of excessive nutrient pollution (eutrophication) that have led to global rises in harmful algal blooms and hypoxia and, thus, alterations in water quality that limit production of young fish. Catastrophic events, especially those owing to human activity in the estuary and watershed, e.g. hydrocarbon spills or massive toxicant releases, are increasingly noted in estuaries and are threats to the sustainability and dependability of reproduction by estuary‐associated fishes.
3.8 Acknowledgements
Thanks are extended to the co‐editors of this volume for encouragement and helpful recommendations. The authors acknowledge their respective institutions for facilities support and for providing dependable online access to library resources. Many field assistants made valuable contributions to research conducted by the authors and cited in this chapter; their names are too numerous to mention. The funders, both governmental and non‐governmental, are thanked for their support of research cited in this chapter. Individual funding agencies are acknowledged in cited papers.
3.9 References
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