The effect of alternate-day feeding on growth and feed conversion in Atlantic cod Gadus morhua

This study aimed to investigate the effect of alternating feeding (Alt) compared with control being fed every day (Con) in the on-growth face of Atlantic cod. Individually marked fish (198 and 98 in the Con and Alt groups, respectively) was sampled for weight and length on 6 occasions over a 15-month period, where mean weight increased from 628 and 758 g to 2635 and 3041 g, for the Con and Alt groups, respectively. Feeding alternate day resulted in 13 percentage more weight gain in the alternating feeding group (2283 vs. 2007 g) and improved feed conversion ratio (FCR, 1.07) compared with control (FCR 1.45). The Alt group consumed significantly less feed (27%) compared with control. The results demonstrate that feeding costs can be drastically reduced without compromising biomass growth by using feeding on alternate days during the on-growing period of Atlantic cod.publishedVersio

Modelling sea lice control by lumpfish on Atlantic salmon farms : interactions with mate limitation, temperature and treatment rules

Atlantic salmon farming is one of the largest aquaculture sectors in the world. A major impact on farm economics, fish welfare and, potentially, nearby wild salmonid populations, is the sea louse ectoparasite Lepeophtheirus salmonis. Sea louse infestations are most often controlled through application of chemicals, but in most farming regions, sea lice have evolved resistance to the small set of available chemicals. Therefore, alternative treatment methodologies are becoming more widely used. One increasingly common alternative treatment involves the co-culture of farmed salmon with cleaner fish, which prey on sea lice. However, despite their wide use, little is understood about the situations in which cleaner fish are most effective. For example, previous work suggests that a low parasite density results in sea lice finding it difficult to acquire mates, reducing fecundity and population growth. Other work suggests that environmental conditions such as temperature and external sea louse pressure have substantial impact on this mate limitation threshold and may even remove the effect entirely. We used an Agent-Based Model (ABM) to simulate cleaner fish on a salmon farm to explore interactions between sea louse mating behaviour, cleaner fish feeding rate, temperature and external sea louse pressure. We found that sea louse mating has a substantial effect on sea louse infestations under a variety of environmental conditions. Our results suggest that cleaner fish can control sea louse infestations most effectively by maintaining the population below critical density thresholds

Exploring the effects of acute stress exposure on lumpfish plasma and liver biomarkers

This study aimed to expand knowledge on lumpfish stress physiology by investigating the effects of acute stress on primary (i.e., cortisol) and secondary (e.g., metabolites) stress responses, as well as oxidative stress biomarkers, from stress exposure to a recovery phase. The results showed that the lumpfish physiological response to 1 min air exposure is mild, in line with recent studies, and comparable to that described for white sturgeons. Cortisol seems to be the most reliable acute stress biomarker in lumpfish, with a significant increase in plasma 30 min after stress exposure, returning to resting levels 2 h after exposure. In contrast, glucose and lactate were not significantly altered by short-term air exposure. Effects on hepatic energy mobilisation were also detected following the acute stress. This study showed that acute 1 min air exposure seems tolerable, allowing a swift recovery. However, more studies on the impacts of air exposure and repeated acute stressors on lumpfish stress and immune responses are required to develop industry standards for lumpfish health and welfare monitoring.publishedVersio

Timing and selectivity of mortality in reared Atlantic cod revealed by otolith analysis

In juvenile fish production, large samples of known-aged material can be sampled at pre-determined time intervals from the same population. This enables an accurate determination of size-selective mortality by means of repeated samplings of fish and comparison of otolith size-at-age based on samples from different dates. An example is provided from an experiment with larval and juvenile Atlantic cod, Gadus morhua, where groups fed smaller sized enriched rotifers did not reveal any size-selective mortality during weaning to formulated feed, while those groups that were fed larger live natural zooplankton lost a significant fraction of the smaller-sized individuals during the same time period. This was contrary to the overall mortality which was higher among the rotifer fed groups during weaning. Part of this difference may be attributed to size differences between the groups, where larger zooplankton fed larvae were more prone to engage in cannibalistic behaviour

Hypoxia tolerance thresholds for post-smolt Atlantic salmon: Dependency of temperature and hypoxia acclimation

Atlantic salmon (Salmo salar L.) experience periodic drops in dissolved oxygen (O2) inside aquaculture sea cages. In order to evaluate whether fish function or welfare is compromised during such drops in O2 (termed hypoxia), it is necessary to establish the limit for acceptable O2 reductions; the hypoxia tolerance threshold. In the present study, effects of temperature (6, 12 and 18 °C) and hypoxia acclimation (33 days of hypoxic periods down to 50% O2 occurring every 6 h at 16 °C) on the routine oxygen consumption rate (ṀO2) and the limiting oxygen saturation (LOS, defined as the hypoxia tolerance threshold) were investigated in undisturbed, fed fish kept in groups with the aim of resembling commercial aquaculture conditions. ṀO2 was measured using open respirometry where a progressive decline in O2 was caused by fish O2 consumption during a period of low water turnover. LOS was defined as the O2 below which fish were no longer able to uphold routine ṀO2. Both ṀO2 and LOS were found to increase exponentially with temperature (Q10 = 2.7 for ṀO2 and 1.8 for LOS), but no effect of hypoxia acclimation was found. The mean (± SE) LOS at 6, 12, 16 and 18 °C was 30 ± 1, 39 ± 1, 47 ± 1 and 55 ± 2% of air saturation respectively. The variation in LOS within and across temperatures was to a large extent explained by variation in ṀO2 (R2 = 0.93), suggesting that LOS increases with any factor that raises the metabolic rate of Atlantic salmon. This study provides aquaculturists and legislators with a lower limit for acceptable drops in O2 at temperatures relevant for the on-growing phase in Atlantic salmon aquaculture, and discusses the variation in LOS that can be expected at a given temperature

Stepwise temperature regulation and its effect on growth, feeding and muscle growth patterns of juvenile Atlantic halibut (Hippoglossus hippoglossus L.)

To investigate the possible direct effect of a stepwise reduction in temperature with increasing size on growth, feeding parameters and muscle growth patterns of juvenile Atlantic halibut (Hippoglossus hippoglossus L.), 804 juvenile halibut (mean initial weight individuals: 14.2 g ± 0.2 SEM) were reared at constant 9, 12 and 15 C or shifted (T-step, i.e. 15–12 C after 36 days) for 99 days. Despite indications of lower optimal temperature for growth with increasing size, equal end weights were obtained between the constant 12 C, constant 15 C and T-step groups. Best overall growth was observed for the group kept at constant 12 C. The limited effect of the T-step group may relate to the size at movement (too big), the temperatures investigated (close to optimum) and the time and size interval investigated (too narrow). Differences in growth were reflected more by alterations in feed intake (CT and F%) than by differences in feed conversion efficiencies (FCE). Differences were found with respect to the density of muscle cells, whereas no differences were found between the average muscle cell diameters. The mean diameter of muscle cells tended to increase only slightly with increasing fish weight, while the mean density of muscle cells tended to decrease. Using an optimum temperature of 12 C, an indication of a possible increased rate of hyperplasia in relation to higher growth was seen

Studies on haemoglobin genotypes in turbot and their relation with growth

Turbot, Scophthalmus maximus, were kept in tanks and weighed every month for about two years. Haemoglobins of 90 untagged and 52 individually Pit tagged fish were analysed by agar gel electrophoresis. A polymorphic system was found in the haemoglobins. The electrophoretic patterns indicated a dimer structure of the proteins. Based on the observations the fish were classified into three phenotypes, supposed to represent three genotypes named; Hb-I(1/1), Hb-I(1/2) and Hb-I(2/2). The phenotype specific growth rate of the fish were analysed using weight data of the individually tagged fish. In addition the final weights and lengths of the untagged fish were analysed for differences in mean size of the phenotype groups