Juvenile growth deficit as an early alert of cockle Cerastoderma edule mortality

X De Montaudouin,M Grandpierre,A Garenne,S Grimault

doi:10.3354/meps13892

Abstract

In the population dynamics of bi-phasic marine invertebrates, the fitness of one stage in the life of a cohort affects that of the following stage. This effect makes the prediction of the fate of a cohort challenging. We conducted a 22 yr monthly survey of a population of cockles Cerastoderma edule focussed on the juvenile stage. Two types of years were distinguished in terms of cohort longevity: a low-mortality group (‘L’) and a high-mortality group (‘H’). The cockle shell lengths of the 0+ cohorts was higher in the L group, in August and September. A cohort could hereby confidently be assigned to the L or H group as soon as August. Maximum cockle abundance in the cohorts, parasite load, or date of recruitment did not discriminate the groups, while an early date for the peak of the 0+ cohort abundance could be related to the L group. The maximum air temperature and the chl a concentration in July were higher in the H group, whereas the minimum salinity during the month of September was slightly lower in the H group of years. Therefore, a juvenile shell length under a given threshold was identified as an early alert for a short lifespan of the cockle cohort. This long-term analysis contributes to the deeper understanding of the population dynamics of bi-phasic invertebrates. The success of juveniles is not solely related to the intensity of recruitment or mortality by predation, but could also be linked to their fitness, their growth rates appearing as a proxy.

Highlights

The fate of bi-phasic marine invertebrate populations is a major concern in the study of population dynamics (Feller et al 1992, Beukema & Dekker 2020)
Two groups of years were defined from cohort abundance monitoring: one set of 9 years when the 0+ cockle cohort survived for at least 1 yr (L, for low-mortality years: 1998, 2000, 2003−2004, 2006−2007, 2009−2011), and another set of 13 years when the 0+ cockle cohort collapsed before the month of November of their first year
Belonging to a group of years did not depend on the cockle abundance during the maximum peak of the cohort (p > 0.05) (Table 1) which ranged between 111 ind. m−2 in 2008 and 5051 ind. m−2 in 2001, both years belonging to the H group

Summary

Introduction

The fate of bi-phasic marine invertebrate populations is a major concern in the study of population dynamics (Feller et al 1992, Beukema & Dekker 2020). When deciphering the life cycle of these species, an important aspect is to understand how each stage of the cycle can have a significant impact on the following stage(s), a process called the ‘cascade effect’ A high adult biomass is not always a guarantee of good juvenile recruitment, and intense recruitment does not necessarily lead to a high number of adults. This reflects the complexity of the stock-recruitment relationship (Beukema et al 2010, Bohn et al 2013, Magalhães et al 2016).

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