Temporal variation in density dependence: Recruitment and postrecruitment demography of a temperate zone sand goby

Spatial and temporal variation in the recruitment of many marine invertebrate and fish species is thought to strongly affect patterns in their demersal abundance. Variation in recruitment of the temperate zone annual goby Sagamia geneionema (Hilgendorf) over eight sand patches within a 1 ha area was investigated during 3 consecutive years (1992-1994). I evaluated the relative importance of recruitment and postrecruitment processes in determining adult density by monitoring the abundance, survival, and growth of juveniles in each patch. In 1992 and 1994, despite pronounced spatial variation in recruitment among sand patches, patterns of recruit abundance established at the end of the settlement season (mid-June or early July) were clearly modified through postrecruitment density-dependent mortality. Consequently, densities in mid-December, when all surviving fish become adults, were stabilized at a low level across patches. The period of mortality which contributed most to stabilizing adult densities was from mid-August to mid-September in both years. In contrast to 1992 and 1994, adult numbers in 1993 directly reflected variation in recruitment success across patches as a result of density-independent mortality, even though recruits were the most numerous in this year. Furthermore, no density-dependent growth was detected in either the juvenile or adult phase. These patterns indicate that, in 1993, recruitment was the major determinant of adult numbers, and competition for resources was unimportant. Thus, the results of this study suggest that both recruitment and postrecruitment processes can influence the demography of demersal populations, and their relative importance can vary from year to year. (C) 1997 Elsevier Science B.V.