Transcript sex small
458 Policies and Their Evaluation Fish 458, Lecture 22 Implementation Uncertainty 458 Implementation uncertainty - how what happens in reality relates to an intended management action. It can be a very major effect (e.g. when catch limits are ignored if they are small). We now consider a range of typical policy types and how implementation uncertainty can be modeled for each. Catch limit Regulation-I 458 Many fisheries are managed by means of the simple decision rule: C a b Stock Size Implementation issues: Low catch limits may be unacceptable for socioeconomic reasons. (Very) high catch limits may be unacceptable due to lack of catching / processing capacity. Large changes in catch limit are highly undesirable. Discarding / high-grading will / may occur. Catch limit Regulation-II 458 Allowing for the first three sources of implementation uncertainty, the decision rule becomes: Ct* a b Stock Size (1 ) Ct*1 ** Ct (1 )Ct*1 * Ct Ct min(Cmax , max(Cmin , Ct** )) if Ct* (1 )Ct*1 if Ct* (1 )Ct*1 otherwise Catch limit Regulation-III 458 To allow for high-grading / discarding, you define two vulnerability functions: that for the total catch that for the landed catch N t 1,a 1 N t ,a s (1 vatot ut ) Ctland wa N t ,a valand ut ; a Ctdisc wa N t ,a (vatot valand ) ut a The catch limit set by the decision rule determines Ctland which in turn determines Ctdisc. Catch limit Regulation-IV 458 Notes: Is often worthwhile allowing for some random error between the catch limit and the landed catch. The vulnerability for the landed catch will be less than that for the total catch for all ages if discarding occurs for reasons other than size (e.g. marketability). Estimating the discard vulnerability function requires data on the discarded component of the catch (this is often not easy to get). Example Policies based on catch limits. 458 Cape hake: based moving the resource to 90% of BMSY. Pacific halibut: 35% of the vulnerable biomass. Australian sharks: based on having an 80% probability of being above the 1996 stock size. Canadian groundfish: based on a fishing mortality rate of F0.1. West coast groundfish: set to move the population towards BMSY. Management Policy for West Coast Groundfish Conduct a stock assessment (generally based on an age-structured model) and apply the control rule: 0.8 0.7 0.6 Catch 458 0.5 0.4 40:10 reduction 0.3 0.2 0.1 Overfished 0 0.00 0.25 0.50 Relative Biomass 0.75 Fishing Effort Policies-I 458 Gear restrictions (gear size, days at sea) are one of the most common forms of management: Australian northern prawn fishery: headrope length is chosen to achieve a fishing mortality of FMSY. Fishing Effort Policies-II 458 Implementation issues: The link between fishing effort and fishing mortality is often very weak. Ignore “effort creep” at your peril – fishers modify their behavior to maximize their returns. Even reducing the number of fishers is expected to increase the average fishing power of the fleet! Enforcement of fishing effort controls is almost as difficult as enforcement of catch limits! Fishing efficiency in Australia’s Northern prawn fishery! 458 5 1993 - the reference year 3 2 5% per annum 1 Cumulative Fishing Power 4 Variable Constant 1970 1975 1980 1985 Year 1990 1995 2000 Size Limits 458 Size limits are commonly employed in invertebrate and sport fisheries (e.g. the size at first reproduction), but: Success depends on released animals surviving. Usually it is not possible to choose a gear type to avoid catching small animals – particularly in multi-species fisheries. Sex-Specific Harvesting-I 458 Basic idea: take males as sperm limitation is not believed to be a major problem for most species. Used primarily when animals are harvested in (relatively) small numbers or selectively for sex or are likely to survive when returned to the water alive. There are several examples of male-only lobster and crab fisheries. However, Alaskan crab fisheries have collapsed despite being male-only fisheries. Sex-specific Harvesting-II 458 When evaluating polices consider the sex ratio of the catches: Minke whales – females are 60+% of the catch as they are closer to the ice-edge. Sex changing fishes pose a particular problem – the large animals may all be males or females. Maximum size limits may help in cases like this. Spatial Regulation-I 458 Spatial management strategies typically involve a system of open and closed areas, possibly with separate catch limits for each area. This could be combined with periodic / rotational harvesting. Spatial regulation is relatively robust to errors in stock assessments (often estimates of biomass may be in error by 100% and occasionally much more). Some species may require “near pristine densities”: Aggregations to spawn. Probability of sex change in sex-changing species. Spatial Regulation-II 458 Issues to consider: Implications of concentrating harvesting in a small area (gear competition, lack of access to sedentary animals). Violation of the closed status of some areas. Mis-reporting of catches spatially. The extent of movement of adults and larvae. Fisheries often operate by finding and fishing aggregations because this maximizes catch rates. General Policy Guidelines-I 458 Keep the biomass above BMSY: Less risky to the target species. Less impact on the ecosystem (although this is often not clear / provable). Greater economic yields are likely given that catch rates will be higher. Note that moving to BMSY may lead to substantial short-term (negative) consequences. These need to be considered along with the benefits of being at or above BMSY. For this reason, yield-per-recruit type analyses may be questionable. 458 General Policy Guidelines-II Employ spatial management to spread the catch spatially. Avoid the problems due to unknown stock structuring: northern cod; Icelandic fin whales. Spatial structuring of a harvesting operation may prevent catch rates providing information about changes in stock size because the harvesters will try to move to keep catch rates high even if stock size is declining. Current Controversies-I 458 No-take zones: These have clear biodiversity / ecosystem benefits (more species, higher densities of target species, larger individual sizes). It is not clear that no-take zones will, however, lead to increased yields, except when the fishery is essentially unmanaged. These depend on the success of enforcement (many small zones versus few large ones). Current Controversies-II 458 Ecosystem management: What does it mean? Tropic interactions, ecosystem functioning, reducing bycatch (of megafauna). Can it be based on models? – are the current generation of ecosystem models too complicated with too many parameters to make reliable predictions? What performance measures should we use to evaluate specific policies (e.g. how to evaluate species of no commercial value)?