Abstract
Fluctuations in marine populations often relate to the supply of recruits by oceanic currents. Variation in these currents is typically driven by large‐scale changes in climate, in particular ENSO (El Nino Southern Oscillation). The dependence on large‐scale climatic changes may, however, be modified by early life history traits of marine taxa. Based on eight years of annual surveys, along 150 km of coastline, we examined how ENSO influenced abundance of juvenile fish, coral spat, and canopy‐forming macroalgae. We then investigated what traits make populations of some fish families more reliant on the ENSO relationship than others. Abundance of juvenile fish and coral recruits was generally positively correlated with the Southern Oscillation Index (SOI), higher densities recorded during La Niña years, when the ENSO‐influenced Leeuwin Current is stronger and sea surface temperature higher. The relationship is typically positive and stronger among fish families with shorter pelagic larval durations and stronger swimming abilities. The relationship is also stronger at sites on the coral back reef, although the strongest of all relationships were among the lethrinids (r = .9), siganids (r = .9), and mullids (r = .8), which recruit to macroalgal meadows in the lagoon. ENSO effects on habitat seem to moderate SOI–juvenile abundance relationship. Macroalgal canopies are higher during La Niña years, providing more favorable habitat for juvenile fish and strengthening the SOI effect on juvenile abundance. Conversely, loss of coral following a La Niña‐related heat wave may have compromised postsettlement survival of coral dependent species, weakening the influence of SOI on their abundance. This assessment of ENSO effects on tropical fish and habitat‐forming biota and how it is mediated by functional ecology improves our ability to predict and manage changes in the replenishment of marine populations.
Highlights
Most marine plants and animals have a bipartite life history whereby reproductive gametes or larvae spend the early stages of their life in the open ocean
Critical processes such as gamete production, recruitment, and growth in fish populations are closely aligned with climatic fluctuations, and the El Niño Southern Oscillation (ENSO) appears to be a key metric in understanding and measuring this climatic forcing on population demographics across a range of tropical and temperate marine taxa (Ong et al, 2016)
We show that juvenile fish abundance, coral recruitment, and the canopy height of macroalgal beds are all positively affected by Southern Oscillation Index (SOI), there is considerable variation in the strength of this relationship according to both taxa-specific and spatial factors
Summary
Most marine plants and animals have a bipartite life history whereby reproductive gametes or larvae spend the early stages of their life in the open ocean. There is, considerable variation in ENSO–recruitment relationships, with the recruitment of some temperate species being higher during El Niño years (Caputi et al 1996), the causes of this variation remain equivocal It is unclear how ENSO-driven shifts in the Leeuwin Current affect tropical species at the northern extent of the Leeuwin Current. To explore why ENSO influences taxa differently, we examined how well reproductive and ecological traits in fish explained variation in the strength and direction of the relationship between SOI and juvenile fish abundance Combined, these assessments of ENSO effects on tropical taxa improve understanding of how large-scale fluxes in climate influence recruitment strength, advancing our ability to predict and manage future marine populations
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