Impact of freshwater input and wind on landings of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) in shelf waters surrounding the Ebre (Ebro) River delta (north‐western Mediterranean)

Abstract

Time series analyses (Box–Jenkins models) were used to study the influence of river runoff and wind mixing index on the productivity of the two most abundant species of small pelagic fish exploited in waters surrounding the Ebre (Ebro) River continental shelf (north‐western Mediterranean): anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus). River flow and wind were selected because they are known to enhance fertilization and local planktonic production, thus being crucial for the survival of fish larvae. Time series of the two environmental variables and landings of the two species were analysed to extract the trend and seasonality. All series displayed important seasonal and interannual fluctuations. In the long term, landings of anchovy declined while those of sardine increased. At the seasonal scale, landings of anchovy peaked during spring/summer while those of sardine peaked during spring and autumn. Seasonality in landings of anchovy was stronger than in sardine. Concerning the environmental series, monthly average Ebre runoff showed a progressive decline from 1960 until the late 1980s, and the wind mixing index was highest during 1994–96. Within the annual cycle, the minimum river flow occurs from July to October and the wind mixing peaks in winter (December–April, excluding January). The results of the analyses showed a significant correlation between monthly landings of anchovy and freshwater input of the Ebre River during the spawning season of this species (April–August), with a time lag of 12 months. In contrast, monthly landings of sardine were significantly positively correlated with the wind mixing index during the spawning season of this species (November–March), with a lag of 18 months. The results provide evidence of the influence of riverine inputs and wind mixing on the productivity of small pelagic fish in the north‐western Mediterranean. The time lags obtained in the relationships stress the importance of river runoff and wind mixing for the early stages of anchovy and sardine, respectively, and their impact on recruitment.

Embedding the effect of environmental conditions on recruitment and survival of the European anchovy (Engraulis encrasicolus): a Bayesian model with dual-time resolution

“…Bayesian models have proven their value for assessing the underlying stock-recruitment relationship of exploited species, along with the associated uncertainty (Dorn, 2002;Mäntyniemi and Romakkaniemi, 2002;Michielsens and McAllister, 2004). In many stocks and particularly in small pelagic fish, the sensitivity of recruitment to the environment impedes the use of traditional stock-recruitment relationships (e.g., Ricker or Beverton-Holt curves) as the unique driver of Figure 2 Posterior distributions (solid lines) for the parameters of the model used to incorporate an environmentally forced recruitment of European anchovy (Engraulis encrasicolus) in the Gulf of Cádiz, Spain.…”

“…Albeit the small RMSE for observed and estimated catches (5 million corresponding to 3.84% of the maximum value of observed catches) can be a consequence of the process simplification induced by the assumption of constant fishing-induced mortality, it shows the goodness of fit of the model in a plausible "What if" scenario: What if catches are proportional to the population, and that scenario is not far from reality according to the results from the previously developed Bayesian model. Although a proportionality between catches and abundance is not necessarily observed and is the subject of hot debate (Pauly et al, 2013), this proportionality is frequently observed in short-lived small pelagic fish (Lloret et al, 2004). The dual resolution also facilitates the embedding of environmental forcing into generalist models, such as the GPDM for fishery management (Mäntyniemi et al, 2015).…”

“…According to Maunder and Deriso (2003), and Män-tyniemi et al (2013), recruitment process error is accounted with a lognormal process:…”

“…Therefore, R t acts as the link between the environmental forcing of anchovy life stages (Ruiz et al, 2009) and the dynamics of the stock as modeled in Mäntyni-emi et al (2013), where the change in total population size equation is defined as…”

“…The physical and trophic environment affects fish stocks and landings (Lloret et al, 2001;Erzini, 2005), particularly for short-lived small pelagic species (Nakata et al, 2000;Guisande et al, 2004;Basilone et al, 2006;Lindegren et al, 2013). This effect is observed at different time scales (Fréon et al, 2005).…”