The diel vertical migration (DVM) and feeding periodicity of myctophids (lanternfishes) were studied in the high-latitude Masfjorden, Norway, and the tropical Red Sea. In Masfjorden, a bottom-mounted echo sounder permitted continuous studies throughout the year, and revealed a diverse seasonal DVM behavior. During spring and summer, when zooplankton peaks in the epipelagic zone, migrating glacier lanternfish performed normal DVM (NDVM), ascending to the epipelagic zone during night and residing below ~200m during daytime. During autumn and winter, when Calanus overwinters between ~150–300 m, migrating glacier lanternfish mainly performed inverse DVM (IDVM), ascending to feed on Calanus in mid-waters during daytime. Non migrating (NoDVM) individuals were present all year below ~300 m in Masfjorden. In the Red Sea, where zooplankton has an epipelagic distribution, the whole population of skinnycheek lanternfish performed NDVM, feeding in the epipelagic zone at night, while residing at ~500–750 m during daytime. The warm waters of the Red Sea were hypothesized to limit the time individuals can stay in the mesopelagic zone without migrating to feed in the epipelagic layers. The DVM behavior of myctophids largely seemed to relate to the distribution of zooplankton, and it was hypothesized that NDVM will prevail with epipelagic distribution of prey, while IDVM and NoDVM are common in areas where zooplankton migrate seasonally to mesopelagic depths. Potential predators were continuously present, found to apparently attack glacier lanternfish, at mesopelagic depth in Masfjorden. Thus, myctophids are under threat of predation even at mesopelagic depth.

Fatty acids (FAs) are used as trophic markers in aquatic food web studies, but few studies have quantified individual variability in FAs profiles over several sites in a range of conditions. I investigated whether FAs profiles of yellow perch (YP), Perca flavescens, vary with body size and between lakes with and without largemouth (Micropterus salmoides), and smallmouth bass (Micropterus dolomieu), the most common and abundant piscivores in lakes of the region. I analyzed the FAs of YP as well as zooplankton, benthic invertebrates, and prey fish collected from eight lakes where bass were either present or absent in the Outaouais region over the summer of 2016. I compared the growth rate of YP between the lakes and the YP in lakes without bass exhibited a slower growth rate. I also compared the FA signatures of YP using redundancy analysis (RDA). 23 FAs could be identified and quantified. FAs profiles were dominated by palmitic- (16:0), oleic- (18:1), stearic- (18:0), and palmitoleic acid (16:1). The RDA analysis based on FAs profiles of YP revealed variation along two main gradients (the presence of bass and the date of capture). The first two eigenvectors accounted for 42.1% of the variation (RDA1=27.6% and 2=14.6%). Arachidonic (20:4) and docosatrienoic (22:3) were the most correlated FAs with RDA1. Due to the sampling period, it was impossible to determine if the observed effects were due to the date of capture, the presence of bass, or a change in metabolism, but the last two were deemed as the most plausible explanations. It was concluded that the utility of FA signatures to quantify diet in natural environments is limited and that FAs might be more successful as markers in primary consumers and other lower trophic levels. It is recommended that a combination of FAs, stable isotopes, and modelling should be used in the future.

Coral reefs are highly complex and also highly threatened ecosystems. Population growth and the unsustainable use of coral reefs have resulted in 55% of the world’s reefs being considered degraded. Fishing, the primary ‘local’ threat on most reefs, has altered the composition of most reef communities. As a result, very few pristine coral reefs remain. Typically, coral reef research is done via underwater visual censuses, providing abundance estimates but no indication of trophic interactions, therefore we know relatively little about the structure of intact reef food webs. Understanding how human activities affect trophic structure and feeding interactions among resident reef species may be important for coral reef conservation. Here, I apply stable isotope analysis to coral reef piscivorous fishes from Kiritimati (Republic of Kiribati), the world’s largest atoll. I examine dietary niche metrics of five focal species (Cephalopholis argus, Cephalopholis urodeta, Aphareus furca, Lutjanus bohar, and Lutjanus fulvus) and of the piscivore functional group as a whole, across an anthropogenic disturbance gradient that results from the atoll’s heavily skewed geographic population distribution. Using bootstrapped stable carbon (δ13C) and nitrogen (δ15N) isotope values, controlled for body size effects and analysed with Bayesian methods using the SIAR (Stable Isotope Analysis in R) program, I provide evidence of isotopic niche differentiation in C. argus and L. fulvus relative to other sampled species in terms of niche width metrics and mean δ13C and δ15N values. I also analyse the effect of fishing pressure at an individual level (controlling for body size effects on stable isotope signatures for each species), population level (accounting for observed differences in body size distributions across the fishing pressure gradient for each species), and the ‘community’ level (accounting for body size and relative abundance differences of the five piscivores across the fishing pressure gradient). These metrics reveal species-specific changes in niche metrics of three of the focal species at the individual level: C. urodeta, showed regionally distinct niche width metrics but no apparent correlation with fishing pressure, while A. furca and L. bohar, both had broader niche width metrics in heavily fished areas. No significant effect of fishing pressure was found at population or community levels. This study provides the first evidence using stable isotopes that fishing can alter the diets of coral reef fishes. The mechanism by which it can do so, while not entirely clear, would most likely be by expanding a given species’ dietary diversity by either forcing it to switch to non-preferred prey items or changing the diet and/or body size of its prey items, both of which would reflect significant ecological changes within a community. This thesis provides evidence of the utility of stable isotope analyses in answering important ecological questions in coral reef food webs, and reveals that fishing can affect reef communities at the most fundamental level of trophic interactions. / Graduate / 0329 / burrilladrian@gmail.com