Influence of offshore transport on small pelagic fish larval dispersion and recruitment in the Western Iberian Margin: A Lagrangian perspective

The recruitment levels of European pilchard (Sardina pilchardus) have decreased in the last decades in the western margin of the Iberian Peninsula, compounding stock pressures and capture collapse. In this work we studied the relationship between offshore transport and European pilchard recruitment in the Iberian Peninsula Upwelling and the role played by Lagrangian Coherent Structures in the offshore larval transport.

Changes in the yearly upwelling regime of the western Iberian margin are thought to be a major factor in the observed decline of recruitment levels, due to their impact on offshore transport patterns. As a proxy for offshore transport, the finite-size Lyapunov exponent is a possible tool to study this phenomenon and identify its root causes. 

The finite-size Lyapunov exponent λ is a measure of dispersion at finite scales, and is defined as:

λ(x, t;δ0,δf) =1/τ*log(δf/δ0),

where x is the position, t is time and δ0 and δf are distance parameters. The variable τ is the time it takes for two particles, initially located at x at time t and separated by δ0 to reach a separation δf. Thus, λ measures the rate of growth of the separation between particles, which depends on the initial position x and time t, in addition to δ0. The use of λ to study dispersion and transport problems in oceanography is well established (Bettencourt et al., 2012, 2017; among others). The ridges of λ(x,t) approximate the Lagrangian Coherent Structures of the underlying velocity field, that are avenues and barriers to transport or eddy boundaries. 

Using recruitment and hydrodynamic model data we established a connection between the recruitment level and the finite-size Lyapunov exponent in the western Iberian margin area and revealed a strong inverse correlation between these quantities in the period 2000-2005 and beyond.