Contrasting Patterns of Larval Mortality in Two Sympatric Riverine Fish Species: A Test of the Critical Period Hypothesis

Understanding the causal mechanisms that determine recruitment success is critical to the effective conservation of wild fish populations. Although recruitment strength is likely determined during early life when mortality is greatest, few studies have documented age-specific mortality rates for fish during this period. We investigated age-specific mortality of individual cohorts of two species of riverine fish from yolksac larvae to juveniles, assaying for the presence of a “critical period”: A time when mortality is unusually high. Early life stages of carp gudgeons (Hypseleotris spp.) and unspecked hardyhead (Craterocephalus stercusmuscarum fulvus)—two fishes that differ in fecundity, egg size and overlap between endogenous and exogenous feeding—were collected every second day for four months. We fitted survivorship curves to 22 carp gudgeon and 15 unspecked hardyhead four-day cohorts and tested several mortality functions. Mortality rates declined with age for carp gudgeon, with mean instantaneous mortality rates (-Z) ranging from 1.40–0.03. In contrast, mortality rates for unspecked hardyhead were constant across the larval period, with a mean -Z of 0.15. There was strong evidence of a critical period for carp gudgeon larvae from hatch until 6 days old, and no evidence of a critical period for unspecked hardyhead. Total larval mortality for carp gudgeon and unspecked hardyhead up to 24 days of age was estimated to be 97.8 and 94.3%, respectively. We hypothesise that life history strategy may play an important role in shaping overall mortality and the pattern of mortality during early life in these two fishes.

阐明决定鱼类补充量成功的因果机制，对野生鱼类种群的有效保护至关重要。尽管鱼类补充强度大概率在早期生活史阶段（此时死亡率最高）被确定，但目前鲜有研究记录该时期鱼类的年龄特异性死亡率。本研究针对两种河生鱼类的不同同龄群开展研究，从卵黄囊幼虫阶段至幼鱼阶段，调查其年龄特异性死亡率，并验证是否存在“临界期（critical period）”——即死亡率异常升高的时段。研究采集了繁殖力、卵大小以及内源性与外源性摄食重叠模式存在差异的两种鱼类：塘鳢（Hypseleotris spp.）与无斑硬头鱼（Craterocephalus stercusmuscarum fulvus）的早期生活史样本，采样间隔为每2天一次，持续4个月。我们为22组塘鳢及15组无斑硬头鱼的四天期同龄群拟合了存活曲线，并检验了多种死亡率函数模型。塘鳢的瞬时死亡率（-Z）随年龄增长而下降，平均瞬时死亡率介于1.40至0.03之间。与之形成鲜明对比的是，无斑硬头鱼的死亡率在整个幼虫期保持恒定，平均-Z值为0.15。研究结果有力证明，塘鳢幼虫在孵化后至6日龄期间存在临界期，而无斑硬头鱼则未表现出临界期特征。截至24日龄，塘鳢与无斑硬头鱼的幼虫总死亡率分别约为97.8%与94.3%。我们推测，生活史策略可能是塑造这两种鱼类早期整体死亡率及死亡率模式的重要因素。