Trine Dale

Rapid scavenging of jellyfish carcasses reveals the importance of gelatinous material to deep-sea food webs

Jellyfish blooms are common in many oceans, and anthropogenic changes appear to have increased their magnitude in some regions. Although mass falls of jellyfish carcasses have been observed recently at the deep seafloor, the dense necrophage aggregations and rapid consumption rates typical for vertebrate carrion have not been documented. This has led to a paradigm of limited energy transfer to higher trophic levels at jelly falls relative to vertebrate organic falls. We show from baited camera deployments in the Norwegian deep sea that dense aggregations of deep-sea scavengers (more than 1000 animals at peak densities) can rapidly form at jellyfish baits and consume entire jellyfish carcasses in 2.5 h. We also show that scavenging rates on jellyfish are not significantly different from fish carrion of similar mass, and reveal that scavenging communities typical for the NE Atlantic bathyal zone, including the Atlantic hagfish, galatheid crabs, decapod shrimp and lyssianasid amphipods, consume both types of carcasses. These rapid jellyfish carrion consumption rates suggest that the contribution of gelatinous material to organic fluxes may be seriously underestimated in some regions, because jelly falls may disappear much more rapidly than previously thought. Our results also demonstrate that the energy contained in gelatinous carrion can be efficiently incorporated into large numbers of deep-sea scavengers and food webs, lessening the expected impacts (e.g. smothering of the seafloor) of enhanced jellyfish production on deep-sea ecosystems and pelagic–benthic coupling.

SENSORY QUALITY OF GONADS FROM THE GREEN SEA URCHIN, STRONGYLOCENTROTUS DROEBACHIENSIS, FED DIFFERENT DIETS

Abstract Dietary protein (<12% to 22%) and carbohydrate (9% to >25%) influences on the sensory characteristics of green sea urchin (Strongylocentrotus droebachiensis) gonads were examined in two feeding experiments, both lasting for 60 days. Gonad indices and sensory quality were evaluated, and were compared with those of gonads from wild sea urchins. Gonad growth was not significantly affected by dietary protein:carbohydrate ratio. Bitter taste was significantly affected by diet in both experiments, sweet taste was significantly affected by diet in only one experiment, whereas the taste attributes old/stale, metal/sour and aftertaste were not significantly affected by diet in either experiment. There was increased intensity of bitter taste with increased dietary protein, and the diet with the lowest protein concentration produced gonads not significantly different in taste from those of wild sea urchins. All sensory attributes related to texture (granularity, firmness, and melting) were affected by diet. Gonads from urchins given the diet with the lowest protein content did not differ significantly from those of wild sea urchins, but there was no consistent relationship between dietary protein and texture attributes.

ROE ENHANCEMENT IN SEA URCHIN: EFFECTS OF HANDLING DURING HARVEST AND TRANSPORT ON MORTALITY AND GONAD GROWTH IN STRONGYLOCENTROTUS DROEBACHIENSIS

Abstract This study addresses effects of handling and air exposure during harvest and transport on mortality and gonad growth of Strongylocentrotus droebachiensis in a proceeding roe enhancement trial. Two experimental factors: (1) handling (gentle and rough) and (2) degree of air exposure (wet and dry) were combined to form 4 different treatments; gentle/dry (GD), rough/dry (RD), gentle/wet (GW) and rough/wet (RW). In the proceeding roe enhancement trial, the highest mortality, exceeding 25%, was observed in the GD treatment. Mortality was 1.5% in RW and RD treatments, whereas no mortality was observed in the GW treatment. Mortality only occurred during the first 4 weeks after harvest. Desiccation appears to be the main cause of mortality. There was a significant increase in gonad index for all treatments during the roe enhancement trial; from 7.6% (median) at the beginning of the trial to 15.7, 14.5, 13.5 and 11.2% (median) at the end for GW, RW, GD and RD respectively. However, the increase in gonad index was significantly lower in the RD compared with the others. The lower gonad growth in RD was probably caused by the high frequency of individuals with visual injuries, a frequency that was an order of magnitude higher than in the other treatments. Overall, there was a clear relationship between visual injuries and gonad index, where individuals with injuries had a significantly lower gonad index at the end of the experiment (median; 10.4%) compared individuals without visual injuries (median; 14.5%). The lower feed consumption and higher feed conversion factor observed in RD indicate that individuals with injuries have a reduced gonad growth caused by a combination of reduced appetite and lower feed conversion efficiency. Regeneration of spines and lesions may have resulted in less resources allocated to gonad growth.

Trap Catch of Green Sea Urchins Strongylocentrotus droebachiensis in Kelp Beds and Barren Grounds on the Norwegian Coast

Abstract Field investigations were performed to find out if trap catch could be an alternative to diving to harvest green sea urchins (Strongylocentrotus droebachiensis). Laboratory experiments in y-maze tubes showed that starved sea urchins responded on tree different baits in about one minute, but they did not discriminate between offal of fish (saithe), sea urchin fodder, and lamina of kelp. The trap catch investigations were performed at 0–2.5 m depth in Ytre Kårvika in Troms County and 2-m depth at Hjelmsøya in Finnmark County in autumn 2002 and summer 2003, respectively. Repeated catches 12 times at the same area in Ytre Kårvika showed higher catches at the last six drags than the first six during one month of fishing. In the Hjelmsøya area highest catches were found in barren grounds with high densities (50–80 individuals m−2) of sea urchins, whereas in kelp beds the sea urchin catches did not exceed catches if baits were not used. Offal of catfish and cod heads caught higher catches than sea urchin feed and lamina of kelp. Ring traps (diameter 45 cm) caught better than dropnets, and dropnets caught better than box traps. The 13 drags with the best catches out of 41 drags caught in mean 1.43 kg trap−1 day−1. Probably during the highest catches, the ring traps were saturated. These traps caught 1.23% of the sea urchin resources in the area per day. Estimates show that an area can be harvested for 41–165 days until it is cleaned, depending on the densities of the resources and the harvesting strategy chosen in the estimates. The traps caught sea urchins with low gonad indices, and the sea urchin industry has to feed them to increase the gonad content before they can be equipped.

DEVELOPMENT OF FORMULATED BAIT FOR EDIBLE CRAB (CANCER PAGURUS L.), USING BY-PRODUCTS FROM THE FISHERIES AND AQUACULTURE INDUSTRY

Abstract Four formulated baits for the edible crab (Cancer pagurus L.), were developed. In all baits, fish skin and saithe were the main ingredients. Fish skin gelatin strengthened by the enzyme transglutaminase functioned as a binder in all baits. In Bait 1, a meal mixture (shrimp meal, kelp meal, fish meal) was added as attractant. In bait 2, protein concentrate (PC; 15% to 30% dry matter, on a dry matter basis >90% protein whereof >50% collagen/gelatin) was added as an attractant in addition to the meals mixture used in bait 1. Blue mussels (Mytilus edulis) and cod (Gadus morhua) roe were used as attractants in Bait 3 and 4 respectively. During three field studies, the perfomance of the formulated baits were compared with that of the most commonly used natural bait, chopped untreated saithe (Pollachius virens). Traps baited with baits 1 and 2 had a significantly lower CPUE compared with traps baited with natural baits, whereas traps baited with baits 3 and 4 had a significantly higher CPUE compared with traps baited with natural bait.