Theory 087/Food & Agriculture /Fisheries technology
Bycatch Follows Gear Curvature
Gear is described by the size it catches. The damage may be done by how gradual the cut-off is.
Bycatch rates are set by the curvature of the selectivity curve rather than by its midpoint, so two gears that catch the same size of target can differ several-fold in bycatch depending on how sharply selectivity changes with size.
01The evidence
Gear modifications change bycatch without changing target catch size, and the differences track the sharpness of selection.
02The tension
Selectivity is summarised by a single size, which predicts bycatch from overlap of size distributions and ignores the sharpness of the curve.
03The mechanism
A gentle selectivity curve catches a wide range of sizes, including many non-target individuals above and below the target. A sharp curve catches a narrow band.
That makes curvature the determinant of bycatch at fixed target size, and predicts that sharpening selectivity should reduce bycatch more than shifting it.
04What this is not
Midpoint design tunes the size caught. This tunes the sharpness, which reduces bycatch without changing the target.
05Predictions
- P1Bycatch should track curvature.
- P2Sharper gears should catch less bycatch at equal midpoint.
- P3Curvature should predict better than midpoint.
06Refutation
Bycatch tracking midpoint at controlled curvature would leave size as the criterion.
07What it changes
It would change gear certification.