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

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 100 000 eggs) at the maximal BMIs observed (~15 mg cm -3 ). Long-term monitoring (>20 yr) of densities of a Wadden Sea M. balth- ica 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 vari- ous 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 pre- dict 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.