Life and death at the bottleneck: causes and consequences of fish mortality rates at the edge of the planktonic phase

Abstract

The causes of mortality in early life stages (ELS) of fish are dynamic through time, space, ontogeny and species. Crossing these factors with exogenous and endogenous mortality sources, even for a single species, may be a task for a lifetime. There is a problem of scale in the analysis of critical mortality processes, as methods of study are fundamentally different between the pelagic phase, the settlement period and the benthic phase. This involved high research costs and likely a loss of synopticity. Reviews on mortality sources suggest that we have done nothing but drawing general drivers and processes, and quantified only a few of those in a reduced number of species. Research on fish ELS has been polarized towards pelagic stages. Whereas the paradigm says that the mortality during these stages is maximal, this does not always mean that the first year class strength is best predicted from variations on mortality at those stages. Mortality drivers and rates may change at a bottleneck defined by the need of many species to settle at the bottom. Around settlement, mortality rates may suffer several abrupt changes. If such events are not identified and the drivers understood, there is little hope for a successful prediction of recruitment variability. Methods of study change as a consequence of a shift in the spatial dimension of the problem. There are fundamental changes in trophism, predator fields and movement patterns. Further, density-dependence may add to the mortality equation. The transition between the pelagic realm and the settlement at a suitable habitat may not be rapid, and shared mortality sources between the pelagia and the benthic realm can coexist for a variable period. In species-rich areas such as tropical systems or the Mediterranean, partition of mortality sources or even the understanding of basic mechanisms become additionally complicated. Many unknowns concerning the mobility of fish around settlement are being resolved through new techniques based on otolith michrochemistry, tagging or inmunology. However, species and system-dependent characteristics may impede the study of mortality processes adequately. I will explore the methodological and conceptual difficulties in estimating mortality sources during this phase and identify key processes and remaining challenges, giving some examples of ongoing projects in temperate areas.