Chris Mullholland, of Otago, asks :-
Having dived the East Coast, why are there only small crayfish found in North Otago and they get bigger as one goes around the south coast towards the Fiordland sounds?
Lucy Jack, a marine ecologist at The University of Otago, responded.
Assuming what you see is the case (i.e. that there are actually more big lobsters in Fiordland than Otago- I don’t have any data for this so I can’t say), I can think of 4 things that could cause this:
(1) The populations could be genetically different. Not the case however, as all NZ lobsters are genetically indistinct.
(2) The environment in which they live might be better in one place, allowing more lobsters to grow larger.
(3) Fishing has the greatest potential to define lobster population structure and whether you are likely to see big lobsters. You may perhaps find smaller lobsters at your dive sites in Otago, because these areas are more accessible and frequented by divers who ‘fish-out’ the big lobsters under recreational quota.
(4) Commercial fishing is the most likely culprit. A historical fisheries concession means that smaller lobsters may be caught commercially in Otago than in other areas. Lobsters from Otago mature at larger sizes than those from Fiordland .The concession, plus the greater size at sexual maturity means Otago lobsters can be caught several years before they are able to reproduce.
I understand that the special fishing concession for Otago was created to support a post-war boom in the export of lobster tails to the US, after the depletion of North Island stocks. The size limit was defined by the size of tail that would allow three tails to be placed in one can at the Dunedin cannery. This isn't based on lobster biology and does not take into consideration the difference in size at sexual maturity between regions (higher in Otago)- a discovery made later than the concession ruling. This concession was designed to be lifted gradually, starting immediately after instigation, but is apparently only now under review.
Although it is now reported to be on the up, the CRA7 (Otago) fishery has been low in recent history and catch-per-unit-effort (how hard fishers must try to catch a lobster) is higher for this area than CRA8 (Fiordland). This might be why it could be harder to find lobsters, and especially big ones, in CRA7 compared to CRA8.