Major changes in sardine growth and body condition in the Bay of Biscay between 2003 and 2016: Temporal trends and drivers

Abstract

In the Bay of Biscay, mean body length and weight of sardines (Sardina pilchardus) have been decreasing since the early 2000s and could severely impact the fishing and seafood industry sector. These trends have no apparent link with fishing pressure, although the latter has been increasing since the late 2000s. As part of an effort to develop suitable assessment and management tools for this stock, we investigated the life-history traits of sardine and analyze its seasonal and inter-annual variations. Based on 14 years of morphometric data from both scientific surveys and professional samples, we analyzed the variability in sardine body condition and its responses to environmental changes. Generalized Additive Models revealed an age-sex specific decreasing trend in body length over the study period, with most of the variability explained by the age class. Linear Mixed Effect Models applied to the body condition evidenced its strong seasonality and an age class specific decreasing trend. Regardless of age class, maximal body condition is reached at the end summer, after the spawning and plankton productive periods. Overall, annual trends in body condition-at-age showed remarkable coherence, with a significant decrease since 2007 for all age classes, suggesting that factors influencing body condition operate at population level. The shift in sardine body condition towards lower values could be broken down into three periods, with a high dependence on surface Chlorophyll-a and sea surface temperature. However, this study highlights that the period supporting the main decrease in body condition is characterized by high Chlorophyll-a, the available proxy for food, which is counterintuitive. Such a result suggests more complex trophic responses involving secondary production, with potential shift in the timing of the production and/or the quality of the food. At the population level, those changes may have a long-term negative effect, with a decrease in body length and important changes in phenology (length at first maturity, reproductive phenology) and potential consequences on sardine population dynamics in the Bay of Biscay. Highlights► Strong dependence on age of both seasonal and inter-annual variability in body condition. ► Sardine body condition strongly declined between 2007 and 2011. ► Increase of temperature and changes in Chlorophyll-a may explain the decreasing trend in body condition. ► High levels of Chlorophyll-a during the critical phase (2007)(2008)(2009)(2010)(2011) suggest that food quality rather than quantity is involved.

Determinism of Temporal Variability in Size at Maturation of Sardine Sardina pilchardus in the Bay of Biscay

“…whereŴ i,s is the predicted body weight of an individual i, of a given length L i,s obtained from a sex-specific length-weight relationship for sex s. A detailed description regarding the computation of the sex specific length-weight relationship used in this study is provided in Véron et al (2020).…”

“…Because Véron et al (2020) emphasized changes in both body condition and body length over the study period and as those variables are known to influence spawning (Lowerre-Barbieri et al, 2011), we investigated potential changes in sardine spawning seasonality over the study period by fitting four models to the data. The first one considered the whole study period (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) while the other three were fitted to each period identified in Véron et al (2020), based on the decreasing trend of body condition (2003-2006; 2007-2011 and 2012-2018).…”

“…Moreover, behind its strong dependence upon body length, sardine maturation process fluctuates among cohorts (explaining 5% and 7% of the total FIGURE 1 | Estimated probability of spawning for three length classes [16 cm (green), 20 cm (blue), and 23 cm (purple)] corresponding to the mean length at ages 1, 3, and 6 over the study period. The first three panels correspond to prediction considering the three identified periods in Véron et al (2020Véron et al ( ) (P1: 2003Véron et al ( -2006P2: 2007-2011and P3: 2012 while the last one represents prediction considering the whole study period.…”

“…While small individuals (13 cm length-class) do not exhibit drastic changes in the proportion of mature individuals under a "threshold" value of body condition (around 1.4), an increase of 0.4 units in body condition leads to a significant increase in the proportion of mature individuals in the 16 cm length-class (from around 25 to 75%, for condition values ranging from 0.7 to 1.1). When this proportion is estimated as a function of length for a small range of body condition (Figure 2B; 0.7, 0.9, and 1.1) corresponding to the median values of body condition observed during the three identified periods (P3, P2, and P1; respectively) in Véron et al (2020), our results show a small increase in the proportion of mature individuals with increasing body condition which corresponds to a small decrease in length at maturation with an increase in condition.…”

“…In particular, Doray et al (2018) highlighted a decreasing trend in both body length and weight for the Bay of Biscay sardine stock without emphasizing any correlations either with local changes in environmental conditions or with fishing intensity. Analyzing this decrease in details, Véron et al (2020) recently noticed a concomitant decline in body condition at the population level. Moreover, recent analyses also suggest a decreasing trend in the proportion of mature individuals at age one which could be related to changes in sardine growth and/or maturation (Véron, 2020).…”