Effects of inconsistent reporting, regulation changes and market demand on abundance indices of sharks caught by pelagic longliners off southern Africa

The assumption of a proportional relationship between catch-per-unit-effort (CPUE)
and the abundance of sharks caught by pelagic longliners is tenuous when based on
fisher logbooks that report only retained specimens. Nevertheless, commercial logbooks
and landings statistics are often the only data available for stock status assessments.
Logbook data collected from local and foreign pelagic longline vessels operating in four
areas off southern Africa between 2000 and 2015 were used to construct standardized
CPUE indices for blue sharks Prionace glauca and shortfin makos Isurus oxyrinchus.
Generalized linear mixed models were used to explore the effects of year, month, vessel,
fleet and presence of an observer on blue shark and shortfin mako variability. Landing
statistics and auxiliary information on the history of the fishery, regulation changes, and
market factors were superimposed on the CPUE indices, to test hypotheses that they
would influence CPUE trends. Indices in the West and Southwest (Atlantic) areas were
elevated for both species, compared to the South and East (Indian Ocean). The scale of
year-on-year CPUE increments, up to an order of magnitude for blue sharks, reflected
occasional targeting and retention, interspersed with periods where blue sharks were not
caught, or discarded and not reported. Increments were smaller for higher value shortfin
makos, suggesting that indices were less affected by unreported discarding. CPUE
indices and landings of both shark species have increased in recent years, suggesting
increased importance as target species. Analysis of logbook data resulted in unreliable
indicators of shark abundance, but when trends were interpreted in conjunction with
landings data, disaggregated by area and month, and with hindsight of market demand
and regulation changes, anomalies could be explained.