MEPS 209:119-129 (2001)  -  doi:10.3354/meps209119

ABSTRACT: The number of eggs produced in the annual spawning season by an adult female Macoma balthica (L.) of Wadden Sea origin increases in a non-linear manner with increasing shell length and body weight. It depends primarily on the weight-at-length immediately before spawning. Lean individuals with a low body-mass index (BMI) of 5.6 mg cm^-3 or less spawn no eggs. Gamete output increases to about one-third of soft-body weight (in the order of 100000 eggs) at the maximal BMIs observed (~15 mg cm^-3). Long-term monitoring (>20 yr) of densities of a Wadden Sea M. balthica population revealed that survival to the spawning season of the next year declined from about 65% at higher-than-average values of BMI (>8 mg cm^-3) to about 30% at the lowest values of BMI observed (5 to 6 mg cm^-3). Annual growth was also significantly lower in years starting with a low BMI value. Because spawning lowers body weight and thus the BMI, spawning is bound to reduce subsequent survival rates and body-weight increments. Thus, a trade-off situation exists between immediate and future spawning outputs. The consequences for total lifetime egg production of various levels of gamete output at first spawning (at an age of 2 yr) were estimated by model calculations, starting from a wide range of body weights at a standard length. The model results realistically predict that maximal lifetime egg production can be achieved only if M. balthica refrain from spawning at BMIs below a certain threshold value (viz. ~6 mg cm^-3, which is close to their actual threshold of 5.6 mg cm^-3) and if their spawning output increases with weight-at-length (again in a manner close to what actually happens). It is concluded that M. balthica populations in the Wadden Sea apply a spawning strategy adapted to the local conditions.

KEY WORDS: Annual survival · Optimal strategy · Reproductive investment · Wadden Sea