The Science of Fishing Down
‘Fishing down’ is the process wherein, when fisheries target all
sizes in an aquatic ecosystem, the abundance of larger
and longer lived fishes with high trophic levels decline relative to the abundance
of the fishes and invertebrates of lower trophic levels. This process, which can
usually be detected through declining trends in the mean trophic levels of the catches
the ecosystem in question, is here illustrated through a variety of regional, national
case studies. We welcome receiving further case studies of 'fishing down'.
The first study to illustrate this on a global scale, and which first coined the
phrase ‘fishing down’ on a large-scale, global basis, was by
Pauly et al. (1998) in Science.
‘Fishing down’ has been shown to occur in a wide range of freshwater
and marine settings. The case studies
documented here are presented as examples to allow for an informed debate of the conditions
under which fishing down can be
masked by extraneous factors,
and to enable an assessment of the intensity of this process, once these masking
factors are accounted for.
masking factors, the following
Taxonomic over-aggregation of catch statistics, where the information
on shifting catch composition is hidden by the coarse categories used to report
Spatial (and bathymetric) expansion of fisheries, wherein the shifted species
composition of catches (e.g., from near-shore, shallow areas) is masked by new catches
from offshore and/or deeper areas.
While there is not much that can be done – other than
gathering disaggregated data – to document ‘fishing down’ in areas where the
catch data are over-aggregated, an indicator has been developed to quantify the
spatial and taxonomic expansion which tend to mask ‘fishing down’ (the
Bhathal and Pauly, 2008).
The further development, application and testing of this indicator
is now an active area of research, both within and outside the Sea Around Us Project.
For a discussion of perceived objections to ‘fishing down’, based
mainly on imputations and misunderstandings, see
The Nature of the Discord.