Statistical analyses of intra‐ and interspecific density effects on recruitment of chub mackerel and sardine in Japan

Density effects of chub mackerel, Scomber japonicus , and sardines, Sardinops melanosticta , on the recruitment strength of each species were examined by multiple regression analyses of year‐class strengths and stock abundances or catch statistics compiled over nearly 30 y. The two species have become successively dominant in the pelagic fish assemblage in Japanese waters during the last few decades. Results suggested that both sardines and chub mackerel played suppressive roles on sardine recruitment (P < 0.01), and it seemed that such inhibition was brought about mainly by the dominant species of the two. A direct density effect on chub mackerel recruitment by either species was not clear, but analyses indicated that both species produced delayed density effects on chub mackerel year‐class strengths (P < 0.01). Year‐class strengths of chub mackerel were highly dependent upon annual egg production, which in turn was apparently affected by the stock abundance of the two species over the previous 2 or 3 y. Delayed intraspecific density effects on egg production seemed to operate strongly until the late 1970s. After the sardine stock increased significantly (i.e. 1980s), however, the per capita annual egg production of chub mackerel did not recover in spite of a decrease in the stock size, possibly owing to the delayed density effects of the sardine stock.