Karin Pittman

Trophic Structure and Community Stability in an Overfished Ecosystem

Gobbled by Gobies A common feature of overfished marine ecosystems is a tendency for biomass to become dominated by jellyfish and microbes, and for the habitat to become anoxic or hypoxic as large fish species are removed. The Benguela ecosystem off the coast of Namibia is a case in point. Utne-Palm et al. (p. 333) describe how the loss of overfished sardines from the Benguela fishery has provided an opportunity for an endemic fish species, the bearded goby, to exploit jellyfish and microbial biomass and to increase in number. These small fish have in turn become the predominant prey species for the larger fish, birds, and mammals in the region. The significance of the goby lies in its ability to forage on resources traditionally regarded as “dead-ends.” The bearded goby has thus become a key stabilizing component to the turnover of energy in the Benguela ecosystem. An endemic goby exploits jellyfish and microbial biomass, partially restoring the food chain in the Benguela ecosystem. Since the collapse of the pelagic fisheries off southwest Africa in the late 1960s, jellyfish biomass has increased and the structure of the Benguelan fish community has shifted, making the bearded goby (Sufflogobius bibarbatus) the new predominant prey species. Despite increased predation pressure and a harsh environment, the gobies are thriving. Here we show that physiological adaptations and antipredator and foraging behaviors underpin the success of these fish. In particular, body-tissue isotope signatures reveal that gobies consume jellyfish and sulphidic diatomaceous mud, transferring “dead-end” resources back into the food chain.

Thyroxine as a mediator of metamorphosis of Atlantic halibut, Hippoglossus hippoglossus

The response of morphological, histological and endocrinological development to exogenous 1-thyroxine (T4) and to water depth during metamorphosis in Atlantic halibut, Hippoglossus hippoglossus, was investigated. Exogenous T4 was given in daily doses of 0.1, 0.05 ppm or a control treatment to halibut larvae at 550 daydegrees (posthatch, premetamorphic) for 14 days. Water depths of 40 cm, 10 cm or 1.5 cm were used to rear halibut larvae from 590 daydegrees for 21 days. Halibut larvae given exogenous T4 at 0.1 ppm had accelerated eye migration relative to MH in fish given 0.05 ppm and in control fish. Pigmentation was correlated with dosage after 14 days. The volume of thyroid tissue was expressed in a dose-dependent manner and exhibited a size-dependency within each treatment. However, the follicles were atypical with reduced colloid, increased lumen and low epithelial cells even in the control group. The results indicate that T4 is a mediator in halibut metamorphosis. In the water depth experiment, only cortisol levels of larvae reared in 1.5 cm water were significantly affected after 21 days, but this was not correlated with metamorphic rate. Hormone profiles, morphological changes and size suggest the existence of a ‘window of opportunity’ for metamorphosis in halibut extending from about 16 mm and tapering off about 21 mm SL. The pooled hormone profiles indicate the commencement of a hormonal cascade similar to that of other flatfishes during metamorphosis. The results indicate that growth, neural and skeletal transformation, and pigmentation are biochemically separate processes in the metamorphosis of Atlantic halibut.

The effect of diet on ossification and eye migration in Atlantic halibut larvae (Hippoglossus hippoglossus L.)

Abstract Larval nutrition affects post-metamorphic phenotype in Atlantic halibut and many deformities are related to skeletal development. In a multidisciplinary study, 24,000 sibling halibut larvae were reared at an average temperature of 11.9 °C on either enriched Artemia or natural zooplankton in duplicate treatments and the development followed. One hundred and eighty sibling larvae up to about climax metamorphosis at 46 days post start feeding (psf) were cleared and stained for bone, and stereological measurements of bone and body area and degree of eye migration were registered. From day 21 psf, the zooplankton-fed fish had more calcified matrix relative to body size than did Artemia -fed halibut and from day 35 psf, this difference was significant (33.7%±1.9 SD vs. 28.6%±4.1, P Artemia -fed larvae at Stages 7, 8 and 9 (pro- and climax metamorphosis), but this difference was significant only at Stage 8 (34.1%±2.56 SD vs. 31.9%±2.32 SD, P

The Molecular and Endocrine Basis of Flatfish Metamorphosis

A significant component of aquaculture is the production of good quality larvae, and, in the case of flatfish, this is tied up with the change from a symmetric larva to an asymmetric juvenile. Despite the pioneering work carried out on the metamorphosis of the Japanese flounder (Paralichthys olivaceus) and summer flounder (Paralichthys dentatus), the underlying molecular basis of flatfish metamorphosis is still relatively poorly characterized. It is a thyroid hormone (TH) driven process, and the role of other hormones in the regulation of the process along with the interplay of abiotic factors are still relatively poorly characterized as is the extent of tissue and organ remodeling, which underlie the profound structural and functional modifications that accompany the larval/juvenile transition. The isolation of genes for hormones, receptors, binding proteins, and other accessory factors has provided powerful tools with which to pursue this question. The application of molecular methodologies such as candidate gene approaches and microarray analysis coupled to functional genomics has started to contribute to understanding the complexity of tissue and organ modifications that accompany flatfish metamorphosis. A better understanding of the biology of normal metamorphosis is essential to identify factors contributing to abnormal metamorphosis.

Rice-fish culture: feeding, growth and yield of two size classes of Puntius gonionotus Bleeker and Oreochromis spp. in Bangladesh

Abstract Concurrent rice-fish culture experiments were conducted in Bangladesh to compare and evaluate the feeding, growth, condition factor, survival and yields of two sizes of silver barb ( Puntius gonionotus ) and tilapia ( Oreochromis spp.) in 1994 and 1995, respectively, and to compare paddy yield and ecological conditions between rice-fish and rice-alone situations. Survival, specific growth rate, condition factor and yields were better with the small-sized (5.6–6.1 cm) P. gonionotus than the larger-sized (8.8–9.6 cm) P. gonionotus and either sizes (4.0–7.5 and 9.5–14.7 cm) of Oreochromis spp. P. gonionotus was found to be a macrophytophagous column feeder with benthic foraging habit on tiny molluscs. There was clear evidence of its foraging on paddy grains. Conversely, Oreochromis spp. was found to be a detritivorous browser or surface grazer. Ontogenetic shifts in diet were pronounced in P. gonionotus while absent in Oreochromis spp. Best mean net fish yield was 271.03 kg ha −1 crop −1 with small P. gonionotus in 78 days culture, significantly higher than with Oreochromis spp. ( P −1 crop −1 in rice-fish and 1.5–1.8 t ha −1 crop −1 in rice-alone culture with insignificant differences between the years ( P > 0.05). Water quality parameters were, in general, within the acceptable range for aquaculture. Small sizes of P. gonionotus should be preferred over the Oreochromis spp. for the integration of rice-fish nursery systems but further work should be done on the dietary importance of rice grains to P. gonionotus .

The influence of first-feeding diet on the Atlantic cod Gadus morhua phenotype: survival, development and long-term consequences for growth

Atlantic cod Gadus morhua larvae reached four-fold (at low larval density) to 11 fold higher body mass (high larval density) at 50 days post hatch (dph) when fed zooplankton rather than enriched rotifers. A short period (22-36 dph) of dietary change affected larval growth positively if changed from enriched rotifers to natural zooplankton and negatively if prey type changed vice versa. Overall survival did not differ between the two larval groups at low larval density, but at high density the rotifer group had a higher overall survival (10.8% v. 8.9%). Long-term growth was affected significantly by larval diet in favour of the zooplankton diet; juveniles reached a 23% higher mass in a 12 week growth period. No difference in growth performance was found between juveniles fed natural zooplankton during the larval period for 36, 22 or 14 days, but all these juveniles performed significantly better compared with the rotifer-fed group. These findings suggest that optimal diet during a short period in the larval period can result in improved growth in both the larval and juvenile period. Improved rotifer quality may, therefore, hold a large potential for growth improvement in this species.

Post-embryonic remodelling of neurocranial elements: a comparative study of normal versus abnormal eye migration in a flatfish, the Atlantic halibut.

The process of eye migration in bilaterally symmetrical flatfish larvae starts with asymmetrical growth of the dorsomedial parts of the ethmoid plate together with the frontal bones, structures initially found in a symmetrical position between the eyes. The movement of these structures in the future ocular direction exerts a stretch on the fibroblasts in the connective tissue found between the moving structures and the eye that is to migrate. Secondarily, a dense cell population of fibroblasts ventral to the eye starts to proliferate, possibly cued by the pulling forces exerted by the eye. The increased growth ventral to the eye pushes the eye dorsally. Osteoblasts are deposited in the dense cell layer, forming the dermal part of the lateral ethmoid, and at full eye migration this will cover the area vacated by the migrated eye. When the migrating eye catches up with the previous migrated dermal bones, the frontals, these bones will be remodelled to accommodate the eye. Our findings suggest that a combination of extremely localized signals and more distant factors may impinge upon the outcome of the tissue remodelling. Early normal asymmetry of signalling factors may cascade on a series of events.

Photoperiodic modulation of metamorphosis in Atlantic halibut (Hippoglossus hippoglossus L.)

Abstract The effect of abrupt changes in photoperiod on induction of metamorphosis in Atlantic halibut was investigated. Larvae were reared in a 3.7-m 3 tank on enriched brine shrimp ( Artemia ) from first feeding at 265 day degrees (d°C) (44 days post hatch, DPH) until 66 days (DPH) under continuous illumination or to a standard length of 17 mm corresponding to the beginning of metamorphosis. Thereafter, the larvae were divided into two groups; one further exposed to continuous light (24L:0D), the other given a 12L:12D regime, with four replicates within each treatment. The experiment was terminated at 116 DPH. An abrupt decrease in photoperiod at commencement of metamorphosis briefly stimulated eye migration and slowed growth. Growth was not significantly different at the end of the experiment. The eye migration to standard length ratio was significantly higher under 12L:12D than in larvae under continuous light at 116 DPH. Continuous light accelerated the appearance of haemoglobin in the circulation, gave a higher frequency of normal pigmentation, and higher survival than did 12L:12D in metamorphosing halibut larvae. The sequence of measurable metamorphic events was generally neural change, growth and skeletal change, circulatory change and finally establishment of the pigmentation pattern. Heterochrony of metamorphic events was observed for eye migration and for the appearance of haemoglobin with respect to age but not size. The results indicate that a short period of decreased photoperiod may act as an initiator for some metamorphic events The underlying mechanism may be the action of photoperiod on melatonin and stage specific thyroid hormone interaction. The interaction between ontogeny, photoperiod and endocrinology should be further investigated.

Body site matters: an evaluation and application of a novel histological methodology on the quantification of mucous cells in the skin of Atlantic salmon, Salmo salar L.

Mucous cell size and distribution were investigated in the skin of five salmon using a novel stereology-based methodology: one (48 cm) fish to test 15 tissue treatment combinations on measures of cell area and density on the dorsolateral region and, using the most suitable treatment, we mapped mucous cell differences between body regions on four (52 cm) salmon, comprising a male and a female on each of two diets. The section site, decalcification, embedding medium and plane of sectioning all impacted significantly on mucous cell size, whereas mucous cell density is more robust. There were highly significant differences in both mucosal density and mean mucous cell size depending on body site: the dorsolateral skin of the four salmon had significantly denser (about 8% of skin area) and larger (mean about 160 μm(2)) mucous cells, whereas the lowest mean density (about 4%) and smallest mean area (115 μm(2)) were found on the head. We found that 100 random measurements may be sufficient to distinguish differences >7 μm(2) in mean mucous cell areas. The results further suggest that salmon exhibit a dynamic repeatable pattern of mucous cell development influenced by sex, diet and possibly strain and season.

Dietary Mannan Oligosaccharides: Counteracting the Side Effects of Soybean Meal Oil Inclusion on European Sea Bass (Dicentrarchus labrax) Gut Health and Skin Mucosa Mucus Production?

The main objective of this study was to assess the effects of 4 g kg−1 dietary mannan oligosaccharides (MOS) inclusion in soybean oil (SBO)- and fish oil (FO)-based diets on the gut health and skin mucosa mucus production of European sea bass juveniles after 8 weeks of feeding. Dietary MOS, regardless of the oil source, promoted growth. The intestinal somatic index was not affected, however dietary SBO reduced the intestinal fold length, while dietary MOS increased it. The dietary oil source fed produced changes on the posterior intestine fatty acid profiles irrespective of MOS dietary supplementation. SBO down-regulated the gene expression of TCRβ, COX2, IL-1β, TNFα, IL-8, IL-6, IL-10, TGFβ, and Ig and up-regulated MHCII. MOS supplementation up-regulated the expression of MHCI, CD4, COX2, TNFα, and Ig when included in FO-based diets. However, there was a minor up-regulating effect on these genes when MOS was supplemented in the SBO-based diet. Both dietary oil sources and MOS affected mean mucous cell areas within the posterior gut, however the addition of MOS to a SBO diet increased the mucous cell size over the values shown in FO fed fish. Dietary SBO also trends to reduce mucous cell density in the anterior gut relative to FO, suggesting a lower overall mucosal secretion. There are no effects of dietary oil or MOS in the skin mucosal patterns. Complete replacement of FO by SBO, modified the gut fatty acid profile, altered posterior gut-associated immune system (GALT)-related gene expression and gut mucous cells patterns, induced shorter intestinal folds and tended to reduce European sea bass growth. However, when combined with MOS, the harmful effects of SBO appear to be partially balanced by moderating the down-regulation of certain GALT-related genes involved in the functioning of gut mucous barrier and increasing posterior gut mucous cell diffusion rates, thus helping to preserve immune homeostasis. This denotes the importance of a balanced dietary n–3/n–6 ratio for an appropriate GALT-immune response against MOS in European sea bass juveniles.