Survival of bycatch species escaping purse seines

Many species of schooling fish are caught using purse seines.

Purse seine

Fish in schools caught by purse seines are generally in aggregations of one species.  However there can also be fish that are too small, too many, or other species such as turtles, porpoise, and tuna that are not designated for capture in the fishery. For management purposes, these non-target animals are considered bycatch and are discarded, either from the fishing gear or from the catching vessel.  

Porpoise and tuna bycatch in purse seine

What are the mortality rates for bycatch species that are discarded or escape from purse seines? Experiments have been designed to answer this question for mackerel that are "slipped" from a purse seine because too many are caught in a net set.

Huse and Vold 2010

Mortality rates of mackerel were related to a stress index of crowding density and time.

Huse and Vold 2010  Stress indices (fish density (kg m⁻¹) times crowding time) from Lockwood et al. (1983) (diamonds) and from our own experiments (triangles). The exponential line is fitted to the data from Lockwood et al.  

Reflex impairment and RAMP can be used in field survival and mortality experiments. Observations of reflex impairment for fishes, turtles, and porpoise can be made in holding nets and related to vitality and mortality using RAMP calculations. Note that the mortality curve for mackerel is similar to RAMP curves for other species.  

Use of RAMP in field experiments and fishing operations can result in large amounts of real time, high quality data on discarded or escaped bycatch vitality and mortality.  This data is key for the effective management of fisheries stocks and conservation of ocean ecosystems.