Effect of protein source ratio and protein concentration in prepared diets on gonad yield and quality of the green sea urchin, Strongylocentrotus droebachiensis

Abstract

Adult green sea urchins ( Strongylocentrotus droebachiensis) were collected from the wild and held in laboratory tanks where they were fed a number of prepared diets or a control of kelp ( Laminaria longicruris and/or L. digitata) for a period of 12 weeks. Twelve different prepared diets were formulated in order to examine two experimental factors: (1) protein concentration at three different levels [19%, 24%, and 29% (percent dry weight of all dietary components)] and (2) protein source ratio at four different levels [rockweed meal, wheat meal, corn meal, soybean meal, fish meal in the following four ratios: 0:30:30:25:15, 15:20:20:35:10, 30:10:10:45:5, 45:0:0:55:0, respectively (percent dry weight of all meals)]. Each concentration was present at each source ratio in a completely crossed experimental design. Gonad colour, percent gonad water, and percent gonad yield of experimental urchins were determined approximately every week while gonad texture, firmness, and taste were subjectively evaluated at the end of the experiment. Results were contrasted with those of wild specimens collected from the source population at weeks 0 and 12 of the experiment. After 12 weeks, all prepared diets produced significantly higher percent gonad yields than kelp-fed urchins or wild controls, the best-prepared diet giving a mean weekly increase of 1.3% in percent yield. Percent gonad yield was not significantly affected by protein concentration, but was significantly affected by protein source ratio; the 15:20:20:35:10 diet gave significantly higher yields than the 0:30:30:25:15 or 45:0:0:55:0 diets (30:10:10:45:5 being intermediate). Gonad colour of urchins fed prepared diets was, generally, pale yellow/orange to yellow-brown/orange-brown at the end of the experiment and not significantly different from kelp-fed urchins or wild controls. Gonad colour was not significantly affected by protein source ratio, but was significantly affected by protein concentration; the 19% and 24% levels produced significantly better coloured gonads than the 29% level. At the end of the experiment, gonad texture of urchins fed prepared diets ranged from smooth to very smooth with distinct gonad segment halves while gonad firmness ranged from firm to very firm. Gonad texture and firmness of prepared diet treatments did not differ significantly from kelp-fed urchins or wild controls and neither protein concentration nor protein source ratio significantly affected gonad texture or firmness. At the end of the experiment, gonad taste of urchins fed prepared diets ranged from bitter to sweet with all prepared diets producing significantly worse tasting gonads than the kelp-fed urchins. Gonad taste was not significantly affected by protein source ratio nor protein concentration, but there was a general trend of worsening taste with increasing protein levels. Based on these results, a 19% protein concentration and a 15:20:20:35:10 (rockweed meal/wheat meal/corn meal/soybean meal/fish meal) protein source ratio are recommended for further dietary research.