The ecology of forage fish species and interactions with marine predators

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The long-term goals of my research program are to investigate the basic ecology of forage fish species and how this shapes the biology of predators in marine ecosystems of northern Canada. Capelin, a small minnow-like fish, is the focal forage fish species that preys on many invertebrates and is in turn preyed on by most large predators, including cod, whales and seabirds in the Northwest Atlantic. Capelin provide essential linkages for energy transfer among trophic levels and, thus, are often thought to mediate ecosystem-level changes. In previous NSERC-funded research, I discovered capelin spawning in deep water (demersal) habitat in coastal Newfoundland, where this species is primarily thought to spawn on beaches. I also provided the first direct evidence that capelin select between warm, beach and cool, demersal habitats based on habitat-specific temperature, suggesting a higher occupation of demersal habitat with predicted climate warming. Although eggs develop normally and larvae emerge from demersal habitat, this habitat is not thought to contribute individuals to the spawning population. My first short-term objective is to elucidate the relative contribution of demersal and beach habitats to recruitment using cutting-edge techniques to quantify otolith microchemistry (i.e. laser ablation inductively coupled plasma mass spectrometry) as a natural tag of natal origin. This requires quantifying habitat-specific otolith chemical signatures (e.g., Sr, Ba) in the core region, representing the period before and immediately post-hatch, by integrating long-term measures of temperature, salinity and water chemistry along with otolith chemistry from larvae raised within each habitat and under controlled thermohaline and chemical conditions in the lab. This will also provide insight into connectivity and metapopulation dynamics, which are critical for effective management and conservation of highly mobile marine fish. As most vertebrate predators rely on capelin as prey in Newfoundland, my second objective is to study the individual-level plasticity of predator foraging strategies to changing capelin density and behaviour. My previous NSERC-funded research revealed that multiple marine predator species aggregate in high abundances (‘biological hotspot’) near a cluster of annually persistent demersal spawning sites, likely owing to augmented prey encounter rates with minimal search efforts. I will integrate recently miniaturized animal-borne tracking devices to examine individual-level foraging behaviour of predators with survey-based shifts in capelin density and behaviour. Survey data also will be investigated to determine predator preferences for different capelin aggregation types to clarify functional relationships between predators and prey. Owing to the high conservation value of this multispecies hotspot, my third objective is to investigate whether similar hotspots associated with capelin demersal spawning sites occur elsewhere in Newfoundland, with the ultimate goal of identifying important areas to protect. Several long-term data sets will be integrated with short-term hypothesis-driven research, forming the basis of many inter-related, field-based student theses. Overall, this will improve the accuracy of capelin year class strength models and allow the development of ecosystem-based harvest rules for this and other forage fish species. This will further develop scientific theories of predator-prey population dynamics to maintain the structure and function of northern marine ecosystems under future fisheries practices and climate change.