Ian D. McCarthy

δ15N and δ13C diet–tissue discrimination factors for large sharks under semi-controlled conditions

Stable isotopes (delta(15)N and delta(13)C) are being widely applied in ecological research but there has been a call for ecologists to determine species- and tissue-specific diet discrimination factors ((13)C and (15)N) for their study animals. For large sharks stable isotopes may provide an important tool to elucidate aspects of their ecological roles in marine systems, but laboratory based controlled feeding experiments are impractical. By utilizing commercial aquaria, we estimated (15)N and (13)C of muscle, liver, vertebral cartilage and a number of organs of three large sand tiger (Carcharias taurus) and one large lemon shark (Negaprion brevirostris) under a controlled feeding regime. For all sharks mean+/-SD for (15)N and (13)C in lipid extracted muscle using lipid extracted prey data were 2.29 per thousand+/-0.22 and 0.90 per thousand+/-0.33, respectively. The use of non-lipid extracted muscle and prey resulted in very similar (15)N and (13)C values but mixing of lipid and non-lipid extracted data produced variable estimates. Values of (15)N and (13)C in lipid extracted liver and prey were 1.50 per thousand+/-0.54 and 0.22 per thousand+/-1.18, respectively. Non-lipid extracted diet discrimination factors in liver were highly influenced by lipid content and studies that examine stable isotopes in shark liver, and likely any high lipid tissue, should strive to remove lipid effects through standardising C:N ratios, prior to isotope analysis. Mean vertebral cartilage (15)N and (13)C values were 1.45 per thousand+/-0.61 and 3.75 per thousand+/-0.44, respectively. Organ (15)N and (13)C values were more variable among individual sharks but heart tissue was consistently enriched by approximately 1-2.5 per thousand. Minimal variability in muscle and liver delta(15)N and delta(13)C sampled at different intervals along the length of individual sharks and between liver lobes suggests that stable isotope values are consistent within tissues of individual animals. To our knowledge, these are the first reported diet-tissue discrimination factors for large sharks under semi-controlled conditions, and are lower than those reported for teleost fish.

Variation in individual food consumption rates of fish and its implications for the study of fish nutrition and physiology

The aim of the present paper is to review recent information on food consumption rates of individual fish and to explore the ways in which values for individual food consumption can be used in studies of fish behaviour, nutrition and physiology. There are two main ways of carrying out nutritional studies in fish in which the aim is to investigate how the amount or the composition of the diet influences growth rate. One involves feeding tanks of animals and measuring the growth rates of the groups. This method stresses the importance of the group response to dietary manipulation and the ground rules for carrying out such studies have recently been clearly expounded (Cho, 1992; Cowey, 1992). An alternative method is to measure the food consumption of the individual fish and to construct from the data on individual animals food consumptiongrowth rate relationships for the species. In some species of fish which can be held and fed individually, e.g. cod (Gadus morhua L.; Houlihan et al. 1989) or minnow (Phoxinus phoxinus L.; Cui & Wootton, 1989), there is not a problem in determining food consumption and growth rate relationships. However, in fish feeding in groups a major problem has been to develop a reliable method to make repeated measurements of an individual’s food consumption. Early attempts involved direct observations of feeding activity or the examination of gut contents in order to estimate consumption. These techniques have proved unsatisfactory, as the methodologies involved are timeconsuming, stressful or invasive and periods of preor postprandial starvation were necessary (for review, see Talbot, 1985). In the 1980s, two non-invasive methods were developed to measure consumption rates of individual fish, held in groups, which employed either feed labelled with the radioisotope I3lI (Storebakken et al. 1981) or with an X-ray opaque particulate marker (Talbot & Higgins, 1983). These techniques permitted repeated measurements of food consumption rates of fish held in groups without any alteration to the feeding regimen. However, for health and safety reasons X-radiography has been the preferred technique (for review, see Talbot, 1985).

Identifying migratory Salmo trutta using carbon and nitrogen stable isotope ratios

Many Salmo trutta populations consist of non-anadromous (freshwater-resident) brown trout and anadromous (sea-run migratory) sea trout. Although adult brown trout and sea trout can usually be identified using differences in size and body colouration, it is not possible to easily identify eggs/alevins as the progeny of brown trout or sea trout. In this study we show that delta(13)C and delta(15)N, measured using a continuous flow isotope ratio mass spectrometer (CF-IRMS), can accurately identify fish eggs as the progeny of freshwater-resident (delta(13)C(egg) = -25.7 +/- 1.9 per thousand,delta(15)N(egg) = 9.2 +/- 1.8 per thousand) or migratory (delta(13)C(egg) = -19.9 +/- 1.1 per thousand, delta(15)N(egg) = 14. 3 +/- 1.5 per thousand) adult female Salmo trutta. Case studies show that stable isotope analysis is a more reliable technique for distinguishing anadromous adult fish than differentiation using morphological characteristics. For example, stable isotope analysis of brown trout from Loch Eck, Scotland, revealed that some individuals possessed delta(13)C and delta(15)N signatures indicative of marine feeding despite visual identification as freshwater-resident fish. It is most likely that these fish are misidentified sea trout although it possible that these fish may be brown trout that have adopted an estuarine feeding strategy to avoid interspecific competition for food within Loch Eck with salmon, powan and Arctic charr. Most stable isotope studies of fish ecology use terminal tissue sampling to provide sufficient biological material for isotopic analysis; however, our study suggests that adipose fin tissue could provide a comparable measure of delta(13)C and delta(15)N. Such a strategy would be invaluable when studying the trophic ecology or migration patterns of fish of high conservation value.

INDIVIDUAL VARIATION IN PROTEIN TURNOVER AND GROWTH EFFICIENCY IN RAINBOW TROUT, ONCORHYNCHUS MYKISS (WALBAUM)

The aims of this study were to examine the relation between consumption, growth and protein synthesis for individual rainbow trout held in groups, and to examine how differences in protein turnover may help to explain the observed differences in protein growth rate between rainbow trout with similar protein consumption rates. Therefore four measurements of consumption were made for 37 individual fish over a 73 day period by using radiography. At the end of the experiment, fractional rates of protein synthesis were measured by using a single flooding dose injection of [3H]phenylalanine. The results showed that fractional rates of protein growth and synthesis increased with consumption rate. However, degradation rates were independent of consumption. Fish consuming similar amounts of food exhibited two- to threefold differences in fractional protein growth rates, and twofold differences in fractional rates of protein synthesis. Nine pairs of fish with similar protein consumption rates but different rates of protein growth were selected to examine if differences in protein metabolism, digestive efficiency or RNA concentration or activity could explain these observed differences in protein growth efficiency. More efficient fish were found to have reduced degradation rates; this was the only significant result obtained. The results support the hypothesis that individual differences in protein turnover are important determinants of growth efficiency in fish.

Does breeding site fidelity drive phenotypic and genetic sub-structuring of a population of Arctic charr?

There is now increasing acceptance that divergence of phenotypic traits, and the genetic structuring that underlie such divergence, can occur in sympatry. Here we report the serendipitous discovery of a sympatric polymorphism in the upper Forth catchment, Scotland, in a species for which high levels of phenotypic variation have been reported previously, the Arctic charr, Salvelinus alpinus. Attempting to determine the proximate mechanisms through which this pattern of phenotypic variation is maintained, we examine the use of the available feeding resource and the genotypic and phenotypic structure of charr in this system. We show clear differences in head morphology between charr from three very closely connected lakes with no barrier to movement (Lochs Doine, Voil and Lubnaig) and also differences in muscle stable isotope signatures and in stomach contents. There were significant differences at 6 microsatelite loci (between Lubnaig and the other two lochs) and very low estimates of effective migration between populations. We conclude that, despite living in effective sympatry, strong genetic and phenotypic sub-structuring is likely maintained by very high levels of site fidelity, especially during spawning, resulting in functional allopatric divergence of phenotype.

Nursery grounds, movement patterns and growth rates of dusky sharks, Carcharhinus obscurus: a long-term tag and release study in South African waters

Knowledge of movement patterns of sharks in coastal waters is critical for the structuring of regional management plans. Through a long-term tag–recapture program, 9716 dusky sharks (Carcharhinus obscurus) were tagged and released along the east coast of South Africa. A total of 648 C. obscurus, principally small sharks ( 200 km that indicated a southerly migration between KZN and Eastern/Southern Cape (E/SC) between June and November. Seasonal northerly migrations were less well defined. The largest southerly and northerly movements were 1323 km and 1374 km, respectively. For sharks moving 1–100 km south from their tagging locality in KZN, an increase in displacement occurred between June and September identifying animals beginning their migration to the E/SC. With increasing displacement, there was also an increase in minimum swimming speed. Calculated growth rates of small sharks of 10.3–11.5 cm year–1 were in agreement with current literature values. Established tag–recapture programs provide an important tool in understanding the ecology of early life-stages of coastal shark species.

Effect of feeding on the tissue free amino acid concentrations in rainbow trout (Oncorhynchus mykiss Walbaum).

This study investigates whether tissue free amino acid (FAA) pools in rainbow trout,Oncorhynchus mykiss (Walbaum), are altered following feeding and the relationships between the amount of food consumed and the FAA pool size. Trout were starved for 7 days to provide baseline data and then refed on day 8. Individual food intake was measured by radiography and the consumption of amino acids (AA) calculated from dietary protein consumption. Total FAA concentrations in the stomach, liver and white muscle were little changed at various times after the meal and this pattern was repeated for the majority of individual FAA. Overall, the most notable change was a reduction in essential FAA concentrations (principally in valine, leucine and isoleucine) in the white muscle following feeding. However, in the caeca total FAA, total essential FAA and a number of individual FAA were significantly elevated at 4, 9 and 15h following feeding. There were few significant correlations between dietary amino acid consumption and total tissue FAA and essential FAA concentration in the stomach, caecum and white muscle; correlations were stronger in the liver. In order to explain the relative constancy of total FAA concentrations in the tissues following food intake (which represents over 100% of the total FAA pool) a model is presented that quantifies the AA flux through the free pools and considers the role of protein turnover in regulating FAA pool size.

Feed utilization efficiencies of Atlantic salmon (Salmo salar L.) parr: Effect of a single supplementary enzyme

Abstract 1. 1. Salmon Salmo salar parr were fed three rations of a control diet and one ration of a diet containing a supplementary enzyme, α-amylase. 2. 2. X-Radiography was used to estimate the consumption rates of individual fish. 3. 3. Consumption-growth relationships for wet weight, dry weight, nitrogen and carbon were established for the two diets. 4. 4. The supplementary enzyme had no statistically significant effect on the consumption-growth relationships or feed utilization efficiency.

Growth and protein metabolism in red drum (Sciaenops ocellatus) larvae exposed to environmental levels of atrazine and malathion

Contaminant exposure can affect development, growth, and behaviour of fish larvae, but its effect on rates of protein synthesis and protein degradation are not known. The aim of the present study was to examine the effects of a single pulsed dose aqueous exposure to environmentally realistic levels of two contaminants, atrazine (0, 40 and 80 microgl(-1)) and malathion (0, 1 and 10 microgl(-1)), on growth and protein synthesis in red drum (Sciaenops ocellatus) larvae. Growth was assessed in terms of increase in length, weight, and protein content over an 8-day period following exposure. Rates of protein synthesis were measured by the flooding dose technique 2, 4, and 8 days after initial exposure to each contaminant by bathing larvae in seawater containing L-[2,6-(3)H] phenylalanine. Exposure to atrazine had no effect on larval growth in length, but caused marginally significant declines in growth in weight (P=0.05) and protein content (P=0.06). However, protein synthesis rates were significantly higher for atrazine-exposed larvae on days 4 (P=0.04) and 8 (P=0.01), suggesting an increase in rates of protein degradation. On day 8, the efficiency with which synthesised proteins contributed to growth was significantly lower (P=0.04) in atrazine-exposed larvae. In contrast, malathion had no significant effects on growth in length or protein content, but there were significant decreases in growth in weight over 8 days. The only other significant effect of malathion was an increase in protein synthesis on day 2 for treated larvae relative to controls. Previous work [Alvarez, M.C., Fuiman, L.A., 2005. Environmental levels of atrazine and its degradation products impair survival skills and growth of red drum larvae. Aquat. Toxicol. 74, 229-241] reported hyperactivity and increased metabolic rate in larval red drum exposed to atrazine, indicating a clear energetic cost. Our results further emphasise the energetic cost of atrazine exposure through elevated rates of protein synthesis and degradation resulting in reduced protein retention efficiency and lower growth rates. Overall, we conclude that exposure to atrazine from surface water run-off can increase the energy requirements and the larval phase duration in red drum larvae, possibly resulting in reduced survival and recruitment in cohorts exposed to atrazine.

The effects of temperature on growth rate and growth efficiency of juvenile common wolffish

The effects of temperature on the survival, growth rate and growth efficiency of larval and juvenile common wolffish, Anarhichas lupus L. were studied at 0–31 days and 9–12 months post-hatching, respectively. The influence of temperature regime during egg incubation on subsequent survival and growth was also examined. The fish were reared at constant water temperatures of 5, 8, 11 and 14°C, and all groups were fed dry pellets. At age 1 month, maximum growth rates were observed at 11 and 14°C. Growth rates and survival of early juveniles were dependent upon incubation history, high growth being obtained only if rearing temperature exceeded the temperature of egg incubation. In juveniles at age 9–12 months, the relationships between temperature and growth, and temperature and growth efficiency were parabolic: the optimum water temperatures for growth (Topt.G) and growth efficiency (Topt.GE) were 11°C and 9.7°C respectively. The growth rate and growth efficiency at these water temperatures were 0.9% day–1 and 0.45 g weight gain per g food offered, respectively.