Contrasting patterns of larval mortality in two sympatric riverine fish species: a test of the critical period hypothesis.

Nicole Mccasker,Nicholas Klomp,Paul Humphries,Shaun Meredith,Howard I Browman

doi:10.1371/journal.pone.0109317

Nicole Mccasker, Nicholas Klomp + Show 3 more

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https://doi.org/10.1371/journal.pone.0109317

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Abstract

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.

Highlights

One of the primary challenges in ecology is to understand the intrinsic and extrinsic factors that influence survival in the early life of animals and plants, when mortality is typically greatest [1,2,3]
The transition from endogenous to exogenous feeding has been traditionally heralded as the ‘critical period’ for fish [5], though more recently it has been suggested that other critical periods may exist within the early life stages, including the embryo stage [7,8], the newly hatched pre-feeding stage [9], and juvenile metamorphosis [10,11]
Ontogenetic development and time to metamorphosis Despite there being little difference in growth rates of larvae over the spawning season, the length of larval duration for carp gudgeon and unspecked hardyhead decreased with the progression of spawning season (Figure 2)

Summary

Introduction

One of the primary challenges in ecology is to understand the intrinsic and extrinsic factors that influence survival in the early life of animals and plants, when mortality is typically greatest [1,2,3]. Critical periods in fish ontogeny have been the subject of many investigations over the last century, conclusions as to both their existence and importance in determining year-class strength remain equivocal [6,12]. This is because of methodological and analytical limitations that include pooling of data across large spatial scales, the effects of immigration and emigration, the patchy distribution of young over temporal and spatial scales, and a lack of mortality estimates that encompass the entire larval period [13,14]. Analytical limitations relate to the absence of comparisons of alternative models that discriminate between constant and varying mortality (but see [15,16,17])

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