Oxygen depletion in coastal seas and the effective spawning stock biomass of an exploited fish species

Environmental conditions may have previously underappreciated effects on the reproductive processes of commercially exploited fish populations, for example eastern Baltic cod, that are living at the physiological limits of their distribution. In the Baltic Sea, salinity affects neutral egg buoyancy, which is positively correlated with egg survival, as only water layers away from the oxygen consumption-dominated sea bottom contain sufficient oxygen. Egg buoyancy is positively correlated to female spawner age/size. From observations in the Baltic Sea, a field-based relationship between egg diameter and buoyancy (floating depth) could be established. Hence, based on the age structure of the spawning stock, we quantify the number of effective spawners, which are able to reproduce under ambient hydrographic conditions. For the time period 1993–2010, our results revealed large variations in the horizontal extent of spawning habitat (1000–20 000 km2) and oxygen-dependent egg survival (10–80%). The novel concept of an effective spawning stock biomass takes into account offspring that survive depending on the spawning stock age/size structure, if reproductive success is related to egg buoyancy and the extent of hypoxic areas. Effective spawning stock biomass reflected the role of environmental conditions for Baltic cod recruitment better than the spawning stock biomass alone, highlighting the importance of including environmental information in ecosystem-based management approaches.

环境条件此前或对商业捕捞鱼类种群的繁殖过程存在未被充分认知的影响，以东波罗的海鳕鱼（Eastern Baltic Cod）为例，该种群正处于其分布范围的生理极限区间内。在波罗的海，盐度会影响鱼卵的中性浮力，而卵浮力与鱼卵存活率呈正相关——唯有远离耗氧主导海底的水层才具备充足溶氧。鱼卵浮力与雌性产卵亲体的年龄/体型呈正相关。基于波罗的海的实地观测，可建立鱼卵直径与浮力（漂浮深度）之间的实测关联模型。因此，本研究基于产卵种群的年龄结构，量化了能够在当前环境水文条件下成功繁殖的有效产卵亲体数量。在1993年至2010年的研究时段内，研究结果显示产卵栖息地的水平覆盖范围（1000~20000平方千米）与溶氧依赖型鱼卵存活率（10%~80%）均存在大幅波动。有效产卵种群生物量这一全新概念，会根据产卵种群的年龄/体型结构，以及繁殖成功与否与鱼卵浮力、低氧区域范围的关联，来考量能够存活的子代数量。相较于单纯的产卵种群生物量，有效产卵种群生物量能更好地反映环境条件对波罗的海鳕鱼补充群体的调控作用，凸显了在基于生态系统的管理框架中纳入环境信息的重要性。