Shrinking fish at risk of being wiped out

Date
    February 1, 2013


AS FISH get smaller under man's environmental impact they will become more prone to predators and a crucial food source will become more endangered than thought obvious, warn scientists.    

Previous research has found some key fish species dwindle in size as larger specimens are trawled out and climate change affects the food chain.   

But until now the broader impact of this shrinking has not been explored.

A team from Australia and Finland used computer simulation to predict what might happen should five species of fish decline in average length over a 50-year period.


The shrinkage was quite small, up to 4 per cent. Yet mortality from predators rose as much as 50 per cent, they found.

The repercussions for catches are significant.

Total biomass for four of the species declined as much as 35 per cent, and catches by the same margin, the researchers say in the Royal Society journal Biology Letters.

''Even small decreases in the body size of fish species can have large effects on their natural mortality,'' the team wrote.

The team looked at five southeast Australian trawl fisheries species - jackass morwong, tiger flathead, silver warehou, blue grenadier and pink ling.

Species biomass decreased for all but the grenadier, which also shrank in size but whose numbers actually rose up to 10 per cent as the fish moved to more coastal areas where it was less vulnerable to predators, according to the simulation.

Man is changing marine ecosystems worldwide - directly through fishing and indirectly through global warming, the researchers wrote. ''Fisheries management practices that ignore contemporary life-history changes are likely to overestimate long-term yields and can lead to overfishing,'' they warned.    

Read more: http://www.theage.com.au/environment/animals/shrinking-fish-at-risk-of-being-wiped-out-20130131-2dnh7.html#ixzz2JapJlQzy

Minimum sizes have a lot to blame for this. Basically we are eating the prize specimens and keeping the runts for breeders. I have seen fishermen attempt to play down the risks associated with this. They appear to have incorporated fisheries management legislation that is less than a generation old into some kind of fishing moral code and will cling to it like a safety blanket, while rejecting all alternative fisheries management tools (eg marine parks) that do not have the same problems and can even help to reverse the trend.

Starvation is the way most top predators die Cods.

We should reduce our reliance on minimum sizes. For some species we should be able to do away with them completely. For example, as we get more no-take zones, more sedentary species will be overprotected compared to the species that move around more. We should be able to reduce or eliminate minimum sizes on these.

One problem with both minimum and maximum sizes is hooking mortality - just because you return it does not mean it survives or breeds successfully. It will however learn not to bite next time.

There is an urban myth that minimum sizes increase total catch in terms of weight because the little fish grow bigger and get caught again. In fact the opposite is true. We would catch more fish, in terms of both number and weight, if we relied less on minimum sizes. One reason is that fish learn. The other reasons are related to ecosystem biology - the little fish end up getting eaten etc rather than growing bigger and being caught again. Growth rate relative to amount consumed also drops. That is one reason why farmers for example sell most of their cattle at one year old, or even younger.

It is the big fish that are the successful breeders. Giving the smaller ones a chance may appeal to a sense of fairness, but it is pretty useless for stock management.

Basically, minimum sizes are about the worst possible fisheries management tool available. People have gotten used to them, taught them to their kids etc and invested emotionally in them.

freediver wrote on Feb 1^st, 2013 at 8:46am:


Like that fine specimen in your nic pic FD? 

fish are smarter than i thought they were

Sure, there is one in the marine park article:

http://www.ozpolitic.com/fish/marine-parks-fisheries-management-tool.html#links

Also, the opening post is about this problem, though admittedly it is a bit vague.

Do you doubt the negative impact of minimum sizes on fish growth rates?

Is that a yes? Do you doubt the negative impact of minimum sizes on fish growth rates?