Ronald Baker

True Value of Estuarine and Coastal Nurseries for Fish: Incorporating Complexity and Dynamics

Coastal ecosystems, such as estuaries, salt marshes, mangroves and seagrass meadows, comprise some of the world’s most productive and ecologically significant ecosystems. Currently, the predominant factor considered in valuing coastal wetlands as fish habitats is the contribution they make to offshore, adult fish stocks via ontogenetic migrations. However, the true value of coastal nurseries for fish is much more extensive, involving several additional, fundamentally important ecosystem processes. Overlooking these broader aspects when identifying and valuing habitats risks suboptimal conservation outcomes, especially given the intense competing human pressures on coastlines and the likelihood that protection will have to be focussed on specific locations rather than across broad sweeps of individual habitat types. We describe 10 key components of nursery habitat value grouped into three types: (1) connectivity and population dynamics (includes connectivity, ontogenetic migration and seascape migration), (2) ecological and ecophysiological factors (includes ecotone effects, ecophysiological factors, food/predation trade-offs and food webs) and (3) resource dynamics (includes resource availability, ontogenetic diet shifts and allochthonous inputs). By accounting for ecosystem complexities and spatial and temporal variation, these additional components offer a more comprehensive account of habitat value. We explicitly identify research needs and methods to support a broader assessment of nursery habitat value. We also explain how, by better synthesising results from existing research, some of the seemingly complex aspects of this broader view can be addressed efficiently.

Nursery Function Drives Temporal Patterns in Fish Assemblage Structure in Four Tropical Estuaries

Despite estuary-to-estuary differences in assemblage composition, fish faunas of tropical Indo-Pacific estuaries show parallel patterns of temporal change, suggesting a common set of ecological drivers. One potentially important driver is the interaction of different patterns of occupancy by functional groups that display different life-history patterns. However, most studies that have considered temporal change lack the detail needed to understand life-history utilisation. Most have focussed on changes in catch per unit effort (CPUE) or probability of encounter, with only one study going further and investigating changes in size structure and then only for a single estuary. One of the reasons for this lack of detail is the large volume of work needed to collect comprehensive data on size structures of species rich assemblages across multiple estuary systems over time. To overcome the logistical limitations on data collection, we used joint patterns of change in CPUE and mean biomass per fish (BPF) as proxies for changes in size structure. We investigated how different life-history strategies contributed to overall temporal patterns of assemblage change across four tropical Indo-Pacific estuaries. The three life-history strategies displayed characteristically different patterns in CPUE and BPF and the relationships between CPUE and BPF that reflect differences in the way that the three groups use estuaries. These different patterns interacted to produce complex assemblage patterns that are likely to be sensitive to location-specific differences in the mix of species from each group, providing at least part of the explanation for the site-specific fish assemblage structures that are characteristic of tropical estuarine fish fauna.

Habitat-specific food webs and trophic interactions supporting coastal-dependent fishery species: an Australian case study

Coastal ecosystems such as estuaries, tidal wetlands and shallow coastal waters are often highly productive and provide important habitats to many recreationally and commercially important fish and invertebrates that use these areas as nursery, feeding and/or reproduction grounds. The diversity of coastlines found worldwide results in differences in types of provisioning and function, and in community structure and trophic organisation. Since almost all coastal fishery species require particular components of the seascapes during specific stages of their life-cycles, it is important to understand the way fish use different habitats throughout ontogeny. Access to rich feeding environments is a key contributor to habitat value, and so knowledge on food webs and feeding relationships, and how these vary over space and time, is central to understanding the importance of the different coastal environments. However, the functional roles of the different habitats in supporting fishery species are still not well understood for most regions. In this study, we review and discuss the available literature to identify key knowledge gaps in the understanding of habitat- and context-specific food webs and trophic interactions supporting fisheries species relying on coastal ecosystems. We use Australia and Australian fisheries species as a case-study, as Australia’s extensive coastline encompasses many of the coastal ecosystems and habitats found globally. Given the ever increasing transformation of coastal landscapes by either direct human action or by sea level rise and changing climate, these knowledge gaps need to be urgently addressed for appropriate management and mitigation of various impacts.

Fish Biomass in Tropical Estuaries: Substantial Variation in Food Web Structure, Sources of Nutrition and Ecosystem-Supporting Processes

Quantification of key pathways sustaining ecosystem function is critical for underpinning informed decisions on development approvals, zoning and offsets, ecosystem restoration and for meaningful environmental assessments and monitoring. To develop a more quantitative understanding of the importance and variation in food webs and nutrient flows in tropical estuaries, we investigated the spatio-temporal distribution of biomass of fish across 28 mangrove-lined estuaries in tropical Australia. We evaluated the extent to which nekton biomass in tropical estuaries responded to spatial and temporal factors and to trophic identity. Biomass was dominated by two trophic groups, planktivores and macrobenthos feeders. Contributions by other trophic groups, such as detritivores and microbenthos feeders, were more variable. Total biomass and the biomass of all major trophic groups were concentrated in downstream reaches of estuaries. The consistent concentration of biomass downstream, and spatio-temporal differences in the contributions by the different trophic groups, indicates substantial differences in food web structure, differences in the contributions from different sources of nutrition and probably unequal flow of productivity into higher levels of the food web in different parts of the estuary. In turn, this suggests substantial qualitative and quantitative differences in ecosystem-supporting processes in different estuary reaches.

Contrasting Seascape Use by a Coastal Fish Assemblage: a Multi-methods Approach

Understanding the range of habitats needed to complete life-cycles is essential for the effective conservation and management of species. We combined otolith microchemistry, acoustic tracking, and underwater video to determine patterns of seascape use by an assemblage of tropical snappers, including two little-known species of high economic importance, the Papuan black bass (Lutjanus goldiei) and spot-tail snapper (Lutjanus fuscescens). All species appeared to have marine larval phases, and post-settlement distributions broadly overlapped across the coastal seascape. However, species and life stages were distributed along a gradient from freshwater to coastal waters. Lutjanus fuscescens is primarily a freshwater species post-settlement, but larger individuals move into brackish estuaries and even coastal waters at times. Lutjanus goldiei appear to recruit to low salinity or freshwater areas. Larger individuals tend to have home-ranges centred on brackish estuaries, while making regular movements into both coastal waters and freshwater. Lutjanus argentimaculatus also ranged widely from fresh to coastal waters, but juveniles were most common in the saline parts of estuaries. Ontogenetic shifts by L. argentimaculatus were similar to those reported from other regions, despite vast differences in the spatial proximity of seascape components. The wide-ranging seascape movements of our target species highlight the importance of maintaining effective connectivity between marine, estuarine, and freshwaters in the region to maintain ecosystem function and support sustainable sport fisheries. The combined approaches resolved some of the ambiguities of individual methods and provide a powerful approach to understanding seascape use by coastal fishes.

Effects of forest width on fish use of fringing mangroves in a highly urbanised tropical estuary

The size and shape of a habitat patch can influence patterns of species utilisation. The present study examined how the width of fringing mangrove habitats affects the composition and use patterns of the fish assemblage using mangrove edge habitats on the flooding tide. Underwater cameras surveyed fish approaching mangrove habitats, ranging from a thin fringe 5m wide to forests over 75m wide, in a highly modified tropical estuary. The fish assemblage composition was similar across all mangrove widths, although the temporal patterns of use varied among mangrove widths for some species. The mean maximum number of individuals in the field of view was similar among mangrove widths, but fish were visible for a significantly greater proportion of time in videos from narrow (<20m) than wide (≥20m) mangrove stands (15v. 3% respectively). At least some fish were visible in the field of view in narrow mangrove fringes throughout the first hour of the flood tide, whereas in wide mangroves areas the presence of fish declined over time. The findings suggest that estuarine fish are using mangrove edge habitat regardless of width, making narrow mangroves viable habitats for estuarine fish.