Not to big, not to small. Harvest slots for fisheries management

When we are fishing, it’s really important that we don’t take out juveniles before they have had a chance to reproduce.  If we take out too many of the young’uns, then the population will decline and eventually collapse.  But how do you know how old a fish or a shellfish is?  Fortunately, there seems to be a ratio between the age of an organism and its size, so fishery managers can put a ‘minimum landing size’ restriction on commercial and recreational fishers.

The minimum landing size system is widely used in freshwater and marine fishery management but there is a problem.  By focusing catches of just the largest (oldest individuals) we risk truncating the age and size of the populations, leaving just smaller and younger individuals in the population.  To try and reduce taking too many of the big guys out of the population, some fisheries have introduces harvest slots, which have both a minimum and maximum landing size.  In a newly released paper, Daniel Gwinn from the University of Florida and colleagues from around the world explore just how well these harvest slots work for freshwater fish targeted by recreational fishers.

Five species were modeled in this study – the Murray cod (Maccullochella peelii) and lake trout (Salvelinus namaycush), Eurasian perch (Perca fluviatilis), arctic grayling (Thymallus arcticus), zander (Sander lucioperca) and the northern pike (Esox lucius).  Each one of these critters differs on a few crucial points.  Some are relatively long lived, some produce many many many offspring.  These factors are crucial in deciding how a fished population may respond to human harvesting pressure. Using some fancy calculations, they assessed how harvest slots and minimum landing size impacts on the population, across different levels of exploitation.  What they found should give some food for thought for fishery managers and recreational fishers alike.

Generally speaking, compared to minimum landing size regulations harvest slots resulted in higher numbers of fish being harvested.  Even better for recreational fishers, the increase in the availability of ‘trophy fish’ (the big fellas – though they would have to return them to the water) and preserved the age structure of the populations better.  Of course no measure is 100% perfect.  Minimum landing size seems to outperform harvest slots when it comes to maximizing yields, particularly when exploitation of the fish was low, and of course fish being harvested would be smaller with a harvest slot because you’re not allowed to land the large fish.  Taking all things into consideration, harvest slots – say the authors – produce the most favourable compromise between conservation and maximizing fishery benefits, perhaps because the two are not necessarily mutually exclusive.

Switching to harvest slots isn’t the ‘magic wand’ that is going to solve our fishery related issues.  As an example, if we take too many fish out of the slot, then very few will make it to the ‘safe’ size range, or if most of the fish caught die anyway then simply not landing them will make no difference to the population the method is trying to protect (that point goes for minimum landing sizes too).

Unfortunately the paper published in the journal Fish and Fisheries is behind a pay-wall, so you will need to either pay or have journal access to view it.  It can be found here dx.doi.org/10.1111/faf.12053

Image:  The fish is a Northern pike – one of the species modeled in this study.  Credit Bugeater/Flickr (CC BY-NC 2.0)

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