Literature review of elasmobranch bycatch and retention rate

To address growing concern over the effects of fisheries non-target catch on elasmobranchs worldwide, the accurate reporting of elasmobranch catch is essential. This requires data on a combination of measures, including reported landings, retained and discarded non-target catch, and post-discard survival. Identification of the factors influencing discard vs. retention is needed to improve catch estimates and to determine wasteful fishing practices. To do this we compared retention rates of elasmobranch non-target catch in a broad subset of fisheries throughout the world by taxon, fishing country, and gear. A regression tree and random forest analysis indicated that taxon was the most important determinant of retention in this dataset, but all three factors together explained 59% of the variance. Estimates of total elasmobranch removals were calculated by dividing the FAO global elasmobranch landings by average retention rates and suggest that total elasmobranch removals may exceed FAO reported landings by as much as 400%. This analysis is the first effort to directly characterize global drivers of discards for elasmobranch non-target catch. Our results highlight the importance of accurate quantification of retention and discard rates to improve assessments of the potential impacts of fisheries on these species.

(Table 1) Depth and time of first appearance of species identified by fish remains in ODP Site 169-1034

Ocean Drilling Program Leg 169S retrieved a complete Holocene sequence from Saanich Inlet, British Columbia, Canada. Fish and diatom remains were extracted from sediments at Site 1034. Very small fish bones, teeth and scales were ubiquitous except in the lowermost glaciomarine clays; scales degraded with depth. In the identifiable fraction, Pacific herring were the most abundant with Pacific hake and cartilaginous fish yielding significant fractions. Fish remains appear just before 12 000 BP but greatest diversity does not occur until about 6500 BP. A smoothed abundance curve highlights two periods of maximal abundance at about 1500 and 6500 BP. Abundances in the last 1000 years are lower than the rest of the record. A correlation with abundances of seven phytoplankton taxa is significant; diatoms explain about a third of the variance. This study demonstrates the use of fish and diatoms from the same paleosedimentary matrix to examine millennia-scale correlations between primary and tertiary production.