Experimental field tests of natural algal diets on gonad index and quality in the green sea urchin, Strongylocentrotus droebachiensis:: a case for rapid summer production in post-spawned animals

We tested whether the roe (gonads) of ''post-spawned'', green sea urchins, Strongylocentrotus droebachiensis, from barren grounds could be enhanced in the field during summer. Experiments were initiated using a low roe-yielding (barren ground) population, which also served as a field control. Specifically, we determined the ability of naturally occurring macroalgae to increase roe yield and enhance roe color relative to field controls. Sixty experimental cages with algae and five test animals per cage (mean test diameter = 50.0 mm) were suspended along the seaward end of a commercial lobster impoundment on Beals Island, Maine, on 18 June 1996. Urchins were fed ad libitum all (mixed diet) or one of four species of macroalgae (Palmaria palmata, Alaria esculenta, Laminaria saccharina and Ulva lactuca). Four replicate cages of each algal treatment and 20 individuals from the control population were sampled (without replacement) every 3 weeks until 20 August. This design permitted the use of orthogonal contrasts in both one- and two-factor ANOVAs. These analyses revealed significant enhancement, relative to controls, of both color and roe yield (gonad index doubled or tripled within 3 months). Algal-fed animals attained a mean gonad index greater than 10%, the minimum commercial standard in Maine, while field populations ranged from 4% to 6%. These analyses revealed differential roe enhancement among palatable seaweeds. The red alga, P. palmata, induced the quickest and highest response. singly, among the four algae tested. Roe production on P. palmata was generally higher, but similar to that of the mixed dirt. The two kelp species. A. esculenta and L. saccharina, consistently produced the lowest yields. Roe yields were correlated with protein levels in the algae. Our study also provides some insight into seasonal allocation of energy and nutrients into gonadal tissue. These data show that off-season allocation to gonadal tissue is biologically feasible in the absence of photoperiodic manipulation and that summer enhancement could be used to meet the off-season (August) market demand Tol roe in Asia. (C) 2000 Elsevier Science B.V. All rights reserved.