Maximizing lifetime egg production in a Wadden Sea population of the tellinid bivalve Macoma balthica: a trade-off between immediate and future reproductive outputs

The number of eggs produced in the annual spawning season by an adult femal e Macoma balthica (L.) of Wadden Sea origin increases in a non-linear manne r 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 tin the order of 100 000 eggs) at the maximal BMIs observed (similar to 15 mg cm(-3)). Long-term mon itoring (> 20 yr) of densities of a Wadden Sea M. balthica population revea led that survival to the spawning season of the next year declined from abo ut 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 als o significantly lower in years starting with a low BMI value. Because spawn ing lowers body weight and thus the BMI, spawning is bound to reduce subseq uent 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. similar to6 mg cm(-3), which is close to their actual threshol d of 5.6 mg cm(-3)) and if their spawning output increases with weight-at-l ength (again in a manner close to what actually happens). It is concluded t hat M. balthica populations in the Wadden Sea apply a spawning strategy ada pted to the local conditions.