Figure 3.23 Weight‐specific growth coefficient (G), mortality coefficient (M) and the ratio G/M for Lateolabrax japonicus five‐day larval cohorts in relation to mean daily temperature experienced by each cohort in the Chikugo Estuary, Japan. G/M is an indicator of productive potential and potential to recruit
(from Shoji & Tanaka (2007b, their figure 5)).
3.7 Summary and conclusions
The patterns of reproduction and ontogeny in the taxonomically rich estuary‐associated fishes from around the world, including all the major groups from elasmobranchs to bony fishes, are as diverse as documented for fishes living in other ecosystems. Further, the diversity in the life histories of bony fishes, the dominant group in estuaries, is complex with egg, larval, juvenile and adult stages having different ecologies. Moreover, some stages that use estuaries also use the adjacent, downstream ocean and upstream freshwaters for parts of their life history. Given these complexities in patterns and modes of reproduction, it is not surprising that recruitment processes of estuary‐associated fishes are also diverse. In the Introduction, we listed six broad topic areas that framed the scope of the chapter. We summarise major conclusions in the six areas.
The state of knowledge of fishes that utilise estuaries for reproduction.There is a substantial foundation of knowledge on patterns and modes of reproduction by estuary‐dependent and ‐associated fishes. Resident, anadromous, catadromous and amphidromous species are represented amongst the diverse taxa utilising estuaries for reproductive and recruitment processes. Demersal and pelagic spawning occur, with a relatively high representation of demersal spawning, particularly by small, resident species. Fecundities range from a few to millions of eggs. Larval stages from spawning by resident and anadromous spawning utilise the estuary as a nursery. Larvae of nearshore and offshore spawners and metamorphosing individuals of catadromous and amphidromous fishes ingress to estuaries. While estuaries often are variable environments that challenge reproductive success, they also are rich and productive ecosystems that support successful recruitment in those species that can tolerate such conditions.
Egg, larval and juvenile stages of fishes that occur in or ingress to estuaries, including ecology, dynamics and behaviour, especially regarding the role of estuaries as nurseries.There is considerable knowledge, but also voids in understanding, of processes that assure success in transport or retention of eggs and larvae, processes that promote ingress to estuaries for species that spawn nearshore or offshore and retention of eggs and larvae within estuaries for resident and anadromous species. For offshore‐spawning species, selection of spawning sites by adults and features of ocean circulation that deliver eggs or larvae to the coast and mouths of estuaries are critical to the recruitment process. Ingress to estuaries by larvae can be passive, but more often is promoted by hydrodynamic processes, combined with sensory cues and behavioural mechanisms that allow larvae to successfully enter an estuary by appropriate use of tides, diel vertical migrations and active swimming behaviour. For eggs and early‐stage larvae within estuaries, retention is required to assure recruitment success. Estuarine residence time and numerous hydrographic processes promote successful retention and, in some cases, up‐estuary transport of larvae.During ontogeny, postflexion‐stage larvae of estuary‐associated fishes often develop substantial swimming ability and behaviours that support ingress to estuaries or retention, in addition to supporting trophodynamics (feeding) processes, habitat selection and predator avoidance. Metamorphosis and associated settlement (by demersal species) represent diverse and complex ontogenetic steps in estuary‐associated species and are particularly notable in the pleuronectiform fishes. In estuary‐associated fishes, shifts in habitats often occur with ontogeny. Such shifts are notable for nearshore and offshore spawners, and particularly so for ocean‐spawning catadromous and freshwater‐spawning amphidromous fishes whose marine larvae ingress to estuaries in the postflexion larval or 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
