While the impact of top-down and bottom-up drivers of ecosystem functions has been given considerable argument, here the mid-trophic level is given focus. In marine systems the influence of mid-trophic level species operates in a ‘wasp-waisted’ structure, where they exert regulatory control by acting as a valve to energy flow between large seasonal pulses of primary production and upper level species. In this study I examine the impact of foraging eastern Pacific gray whales (Eschrichtius robustus) on mysid species at the ‘wasp-waist’ (Order Mysidacea), and vice versa, at feeding sites in Clayoquot Sound off the west coast of Vancouver Island. I appraise previously unknown aspects of the ‘prey-scape’, and further explore life-history traits that allow prey populations to persist in a given species array. The set of problems that I examine are all based on the whales’ top-down forcing in a localized area, and the prey response. I use several scales of observation as dictated by the nature of each question. I examine top down forcing and subsequent prey switching over a 25-year period, the variation in foraging intensity over a 15 year period, the differential prey species’ response to persistent predatory pulses that creates dominance and diversity among the mysid species flock, and whales’ within-season response to possible satiation. Each of these studies is linked by the common goal of illuminating the intimate relationship between predator and prey. Gray whale foraging has decimated amphipod prey resources in the study area past the point of recovery over the last 25 years, and the prey resource is no longer a viable energy source. This has led to the abandonment of benthic-feeding by gray whales in the area, and a switch to mysids as a primary prey source. It is in investigating these mysid species’ ability to rebound following severe foraging pressure that I uncovered two principal life history strategies, one held by the single dominant mysid species, and another used by 9 or 10 others consistently sampled. The capacity for renewal of mysid swarms is imperative if Clayoquot Sound is to persist as a productive foraging area for gray whales. The pattern of this relationship that I present, based on a 15 year span, was previously unknown. Intense foraging of mysids by gray whales during a summer affects the reserves for the following season, leading to a biennial fluctuation in the number of whales the area can sustain, although some of the heaviest foraging seasons require several years to show mysid recovery. I state 9 or 10 other species, as through the intense examination of mysids here, there may be a new species designated. The data gathered by myself and colleagues over the past 25 years that whales have been studied in Clayoquot Sound, clearly shows that predation by baleen whales can affect the future quality of their foraging areas, as well as influencing the population, life-stage and diversity of prey species. My work furthers knowledge in life history characteristics of the mysid species present in the study area, particularly growth and reproduction, and ability to capitalize on a release of predation pressure over winter to recover. That, in turn creates a series of following questions about how different life history strategies make use of a variety of possible energy pathways to stabilize ecosystems at least at discrete spatial scales. / Graduate

Mysid shrimps (Mysidacea, Crustacea) are efficient zooplanktivores in both marine and freshwater systems as well as lipid rich prey for many species of fish.  Although some efforts have been made to study the role of mysids in the Baltic Sea, very few studies have been carried out in recent time and there are still knowledge gaps regarding various aspects of mysid ecology. This thesis aims to explore some of these gaps by covering a mixture of topics. Using multifrequency hydroacoustics we explored the possibility to separate mysids from fish echoes and successfully established a promising and effective method for obtaining mysid abundance/biomass estimates (paper I). An investigation of the current mysid community in a coastal area of the northern Baltic proper (paper II) demonstrated that the formerly dominant, pelagic mysid Mysis mixta had decreased substantially (~50%) in favor for phytoplanktivorous, juvenile Neomysis integer and Mysis relicta sp. By examining different aspects of mysid behavior, we studied the vertical size distribution of mysids in the field and found that size increased with depth/declining light, irrespective of temperature; indicating that their vertical size distribution primarily is a response to predation (paper II). In paper III, a combination of ecological and genetic markers was used to investigate intraspecific differences in migratory tendency. Both marker types indicated that some part of the Mysis salemaai population is sedentary on the bottom and that this strategy is a phenotypically plastic but persistent trait, analogous to the partial migrations seen in many birds and fishes. In paper IV a temperature and weight specific respiration model was developed for the littoral Praunus flexuosus. Routine respiration was moreover elevated by post-prandial effects (specific dynamic action) for longer times than previously suggested. Consequently, ignoring such effects could significantly bias respiration measurements. / At the time of doctoral defence the following papers were unpublished and had a status as follows: Paper  2: Accepted; Paper 3: Submitted; Paper  4: Accepted

Gray whales (Eschrichtius robustus Lilljeborg) forage predominantly on hyper-benthic mysids (Mysidae) along the southwest coast of Vancouver Island, British Columbia. The role of mysids in the abundance and distribution of foraging gray whales prompted interest in this study. I relate the inter- and intra-annual foraging behaviour of gray whales to the number of mysid patches and biomass through boat based surveys of foraging whales, and the application of acoustic techniques for estimating mysid prey. I examine the spatial pattern of foraging gray whales and the 10 m isobath. The abundance and distribution of mysid patches are more common at a water depth of 10 m, and the likely mechanism driving the spatial pattern between foraging gray whales and the 10 m isobath. I examine whale abundance and distribution patterns during three consecutive foraging seasons. More whales forage in years when mysid prey are more abundant, and mysid patches are of larger size and higher in biomass. Whales have a considerable top-down effect on mysid populations. Years of heavy whale predation depletes mysid stocks. Mysid populations tend to increase in years of low whale activity. I examine whale abundance and distribution patterns of habitat use during a foraging season along the southwest coast of Flores Island and Nootka Sound. The abundance of mysid patches and biomass in Nootka Sound does not influence the whales’ use of Flores Island; rather the whales exploited both areas concurrently. This study expands the understanding of gray whale habitat use along the southwest coast of Vancouver Island, British Columbia.

Gray whales (Eschrichtius robustus Lilljeborg) forage predominantly on hyper-benthic mysids (Mysidae) along the southwest coast of Vancouver Island, British Columbia. The role of mysids in the abundance and distribution of foraging gray whales prompted interest in this study. I relate the inter- and intra-annual foraging behaviour of gray whales to the number of mysid patches and biomass through boat based surveys of foraging whales, and the application of acoustic techniques for estimating mysid prey. I examine the spatial pattern of foraging gray whales and the 10 m isobath. The abundance and distribution of mysid patches are more common at a water depth of 10 m, and the likely mechanism driving the spatial pattern between foraging gray whales and the 10 m isobath. I examine whale abundance and distribution patterns during three consecutive foraging seasons. More whales forage in years when mysid prey are more abundant, and mysid patches are of larger size and higher in biomass. Whales have a considerable top-down effect on mysid populations. Years of heavy whale predation depletes mysid stocks. Mysid populations tend to increase in years of low whale activity. I examine whale abundance and distribution patterns of habitat use during a foraging season along the southwest coast of Flores Island and Nootka Sound. The abundance of mysid patches and biomass in Nootka Sound does not influence the whales’ use of Flores Island; rather the whales exploited both areas concurrently. This study expands the understanding of gray whale habitat use along the southwest coast of Vancouver Island, British Columbia.