Influence of wind events on larval fish mortality rates in the southern California Current Ecosystem

Abstract

Wind-induced mixing can affect the vertical distribution of plankton in the upper water column, influencing the prey available for larval fishes. The stable ocean hypothesis proposes that periods of calm winds facilitate the development of plankton layers at concentrations sufficient for successful larval foraging and increased survival. Conversely, storm events redistribute prey, leading to reduced foraging success. Here, we investigate this hypothesis by comparing larval fish mortality rates estimated from 37 years of ichthyoplankton data against metrics of wind events defined as storms and calm periods. Contrary to expectations, we found that mortality for Pacific hake (Merluccius productus) significantly decreased as storm events increased in the southern California Current Ecosystem. Mortality rates for northern anchovy (Engraulis mordax), Pacific sardine (Sardinops sagax), Pacific mackerel (Scomber japonicus), and jack mackerel (Trachurus symmetricus) had no relationship to storms, and no species’ mortality rates were related to the number of calm events. Our results highlight the differing sensitivities of larval survival among fishes in the region and indicate that responses to atmospheric processes are species-dependent.

Stage‐specific drivers of Pacific hake (Merluccius productus) recruitment in the California Current Ecosystem

“…However, model diagnostics for these models were poor, indicating that they were likely overfitting the data. Increased storm duration and the number of calm periods were likely linked with higher recruitment due to the previously described mechanisms, with enhanced feeding as prey are more frequently mixed downwards or descend to avoid turbulence during storms (Turley & Rykaczewski, 2019). An alternative mechanism could be that increased turbulence associated with increased storm duration would disrupt potential predation on larval hake as contacts rates decreased with high turbulence (Landry et al, 1995, and references therein).…”

“…For example, Peterman and Bradford (1987) found that the mortality rate of northern anchovy larvae declined as the frequency of calm periods with low wind speeds increased. However, Turley and Rykaczewski (2019) found that the number of hake recruits per spawning stock biomass was negatively correlated to the number of distinct calm periods per spawning season and that larval mortality significantly decreased as the number of storm events increased. While storm‐induced mixing can disrupt or dilute patches of plankton, the authors suggested that these negative effects could be offset by increased contact rates between first‐feeding larvae and their prey (e.g., MacKenzie et al, 1994; MacKenzie & Leggett, 1991).…”

“…While storm‐induced mixing can disrupt or dilute patches of plankton, the authors suggested that these negative effects could be offset by increased contact rates between first‐feeding larvae and their prey (e.g., MacKenzie et al, 1994; MacKenzie & Leggett, 1991). Turley and Rykaczewski (2019) also postulated that larval hake at the base of the mixed layer could benefit from turbulence avoidance behavior by prey in the mixed layer, with prey becoming more susceptible to predation as they swim downward (Franks, 2001). These mechanisms could also explain the negative correlation found between recruitment and the number of days between storm events in our study, as more frequent storm events would maintain a downward flux of surface prey, leading to increased encounter rates, higher growth and survival of first‐feeding hake larvae, and higher recruitment.…”

“…A calm period was identified when the wind speed was below 10 m s −1 for a minimum of 10 days, and wind speeds were above the same threshold during the preceding 18 h. A minimum of 10 days was used because Pacific hake yolk‐sac larvae must find food within this period, or irreversible starvation occurs (Bailey, 1982). Further methodological details can be found in Turley and Rykaczewski (2019).…”

“…Periods of calm are associated with vertical stratification of the water column, aggregating prey in sufficient concentrations to support successful foraging, growth, survival, and recruitment, while storm events can disperse both larvae and food patches, leading to lower foraging success ("Stable Ocean" hypothesis, Lasker, 1978Lasker, , 1981. We included indices for the mean number of (CALM larv , STORM larv ), duration of (CALMD larv , STORM-D larv ), and time between (CALMB larv , STORMB larv ) distinct calm periods and storms events from February to May using the methodology outlined in Turley and Rykaczewski (2019). Wind events were identified using modeled wind output available from the NOAA National Centers for Environmental Prediction Climate Forecast System Reanalysis model (Saha et al, 2010) for the region between 28 and 36 N. Storm periods were identified as intervals when the wind speeds were equal to or greater than 10 m s À1 for a minimum of 18 h (and below this threshold for the preceding 96 h).…”