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Title: Gastric emptying, food consumption and ecological impact of whiting, Merlangius merlangus (L.) in the eastern Irish Sea marine ecosystem
Author: Seyhan, Kadir
ISNI:       0000 0001 3394 7506
Awarding Body: University of Wales, Bangor
Current Institution: Bangor University
Date of Award: 1994
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Gastric emptying of Sprattus sprattus and Arenicola spp. (gastric lavage) and formulated pellets (X-Ray studies) was investigated in a range of sizes of whiting, Merlangius merlangus fed different meal sizes at different temperatures. Emptying of indigestible solids, barium sulphate spheroids and ballotini were also studied. Additionally stomach contents of whiting sampled from Red Wharf Bay, Eastern Anglesey, UK. were analysed to determine diet composition, diel feeding periodicity and feeding strategy of whiting. Daily and total annual intake of main prey taxa, sprat, sandeel and crab, were estimated. Finally feeding behaviour of whiting in captivity was monitored. With respect to the studies conducted under laboratory conditions, it was found that gastric emptying is best described by a linear function. The results indicated significant changes in gastric emptying rates with both prey type and size. The seasonal change in temperature did not yield a significant change in gastric emptying rate. Increase in diet surface area also did not alter the gastric emptying rate significantly. The results have also shown that in continuously- feeding whiting gastric emptying is affected by the second meal such that the arrival of the second meal is accompanied by rapid emptying of a small fraction of the first meal. Despite difficulties encountered with individual variation among whiting of similar size held under similar conditions, gastric emptying times (GET) for both natural food and formulated pellets can be adequately predicted by the equations: GET = 126.47 W -0.111 g 0.26 e -0.068T for the natural food, GET = 6.6 e -0.09T + 2.48 W0.566 e -0.044T for the formulated pellets; where W is the fish weight (g), S is the meal size (g) and T is the temperature (°C) suggesting that a similar meal size of formulated pellet is emptied faster than natural food, Sprattus sprattus. This was interpreted as evidence that natural diets may reflect a combination of 2 factors (a) the relative size of stimulation to the stomach (% distension) and (b) the existence of protective, fibrous skin which resists digestion. Barium sulphate spheroids of diameter 1 mm (ca 20 per g food) and ballotini of diameter 0.029- 0.049 mm (60-100 per 1.58g) were selectively retained by the whiting stomach, suggesting that these indigestible materials can not be used in gastric emptying studies in whiting. Under laboratory conditions with continuous food availability, medium size whiting (155.25±27.98 g) consumed 5.29 g sprat corresponding to 3.41 %bw (approximately 3 sprat, 1.88g each) at 14 °C. The return of appetite after a single meal was found to follow the gastric emptying curve closely. However if food is not offered directly, but made available through demand feeding, the whiting feed rhythmically (every 21 hours), under these conditions it was estimated that feeding activity returns when the stomach is 40 % full. It wa§ found that, in the wild, whiting prefer fish (Sprattus sprattus and Ammodytes spp. ) and crustaceans and only turn to the polychaetes as a third option, even when they are readily available, when preferred food is scarce. The availability of prey was found to be the main criterion in whiting feeding strategy. In March fish were dominant for most whiting, in August and September/October however, Liocarcinus spp. and Corystes cassivelaunus were the main prey items found in whiting stomach reflecting the availability of these prey species. A highly correlated relationship was found between predator (whiting) and prey (sandeel) length, however this was not noticed for sprat; whiting prey on sprat regardless of its length. An increase in feeding intensity was observed in the morning (August), in the evening (September/October), or at both times, crepuscular feeding pattern, (March). However, when feeding behaviour was investigated under laboratory conditions it was found that whiting is mainly a day time feeder with a slight increase in the morning and in the evening, but some feeding also occurs at night. Additionally a strong tendency was found for crustaceans to be found in fish captured during the daylight hours and fish at night in August and September/October. A linear model to estimate food consumption from the field samples was developed. F= (S2-S1) + 1.5 K T, where S2 and SI are average stomach contents at time tI and t2, K is the gastric emptying rate (gh-1) and T is temperature (°C). The average recruited whiting stock in the area of approximately 150 km2 in Red Wharf Bay was estimated to be 129,000 with a total biomass of 22 tonnes. It was estimated that young whiting (groups 0+ to 3+ years) in Red Wharf Bay eat between 1.29 and 6.57 g day-1 in February. By August, when temperature has maximised, these values increase by approximately 70-80 %. Daily intake of sprat, sandeel and crab was estimated to be 0.41,0.14 and 0.14 g for the 0+ group. This was increased to 0.70,0.80 and 0.94 g by the age of 4. On a daily basis the long-term average of the amount of sprat, sandeel and crab consumed were estimated to be 820, 370 and 520 g/km2/day respectively. Total food consumption per year by the recruited whiting stock in the area studied however was 44.8 tonnes of sprat (approximately 11.5 million sprats), 20 tonnes of sandeel and 28.5 tonnes of crab. These figures were compared with the available data from the North Sea as well as from other parts of the Irish Sea and it was concluded that whiting in the Irish Sea eat more than they do in the North Sea.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Fish feeding rates Zoology Aquaculture Fisheries