Abalone FAC Block Caps sub-committee meeting Thursday May 17.
Abalone FAC Block Caps sub-committee meeting Thursday May 17.
In order to put in place a block cap, we first need to decide what is an acceptable catch (Sustainable catch target) for a particular block, and then, to decide what level of catch over that target is acceptable without risking long-term damage to the stocks in that block. Therefore the three issues requiring discussion and agreement are;
1) Objective process for determining “Sustainable catch target”
2) Objective process for deciding when to take management action
- excessive catch = cap
- insufficient catch
3) Agreed management action
1) Objective process to define “Sustainable catch target”
Ideally we would use a length-based predictive model to assist with setting a sustainable catch target. This would require detailed growth data for each block, which we do not have, and could not afford to collect.
Out only real alternative is to use the catch history for each block to arrive at a “Sustainable catch target”. There are some issues with this approach that we need to keep in mind;
The reefs and stocks fished now, may have changed considerably through time
The catch in each block may have varied due to changes in TACC’s, size limits, zonation, market preferences, diver preferences and profitability of fishing in remote areas, in addition to natural fluctuations in stocks in that block. In some years the catch taken may be well in excess of the sustainable catch, while in others (for reasons above), it may be well below. For any one year however, we cannot reasonably determine whether the catch was sustainable for that year.
Options for calculating “sustainable catch target” for each block using our historic catch data.
1) Mean annual catch using all years
2) Mean annual catch using the inner 90 percentile of years
3) Mean annual catch, restricted to years within the quota management system
4) Mean annual catch of QMS years using the inner 90 percentile of years
N.b. The inner 90 percentile of years has the effect of excluding the top 5% of years (= 2 highest years) and the bottom 5% years (= 2 lowest years). We could also choose a different percentile e.g. 80, if that was deemed appropriate
2) Objective process for deciding when to take management action
Having decided on a sustainable catch target, we need to decide what catch above or below that will trigger a management action. In the past we have used a figure arrived at by general agreement, relying on our knowledge of the fishery in that block. This subjective approach is difficult to defend, and an unbiased, objective process for determining the “cap” is preferable.
Having calculated a mean catch, one option available is to use a confidence limits approach, which takes into account the variation among years used to calculate the mean, and identifies a cutoff point where a value is different to the mean at a given level of confidence (ie. 1 in 20 chance of being wrong = 95% CL). This has the benefit of being unbiased, and is also strongly linked to the probability that the value (year) is significantly different from the mean annual catch used.
Typically a 95%CL is used, but it is equally acceptable to use 90%, 80% or 75% if we can define the risk we are willing to take. A 90%CL = risk of 1 in 10 of getting it wrong, and an 80%CL = a 1 in 5 chance of getting it wrong.
BUT: Having calculated the 95%CL for the mean annual catch, my concern is that this value is two restrictive, and if we use this approach we may want to argue the merits of using a 90% or 80% confidence limits. [Update: have actually used 99% CL, as this allows catch to fluctuate within about 10% to 20% of the SCT]
3) Agreed management action
This is the hardest of the three tasks.
Some options are;
1) non-negotiable action – e.g. cap enforced
2) non-negotiable action – triggers review process (could become subjective i.e. what do you reckon, she’ll be right!!)
3) negotiable action (well .. why did we bother with the cap?)
A variation on 2) seems most appropriate for example.
High Catch cap is reached – triggers review
If catch rates are high and increasing - allow small addition catch prior to closure
If catch rates are low and increasing - close block.
Low Catch cap at end of year – triggers review recommending;
a) Research surveys
b) diver interviews of fishers active in that block
Craig Mundy Page 1 23/03/2010