You may need to clarify a few points. Does your question refer to the catch
rates (i.e. CPUE) of sharks or are you referring to BRD-imposed change on
incidental fishing mortality of sharks? Because your question refers to
populations of sharks I assume you are really interested in incidental
mortality. Incidental CPUE of sharks may not equal incidental fishing
mortality, unless you have such supportive data (i.e., 100 incidentally
caught sharks equals 100 incidentally killed).
Are logbook data available for this fishery? One way that you can
demonstrate benefits from BRDs is to extrapolate upwards using fishing
effort, if you have access to the particular fishery's catch and effort
database. For example, if you know that before this particular BRD was
developed and tested, that the incidental catch rate of sharks in this
fishery in question was 10 sharks per day of effort (you provided no details
about the effort for this fishing, so it may be in per meter net set per
day, or perhaps per boat-day), and that there were, on average 1000
effort-days in the fishery annually, then the before-BRD annual catch rate
was (10 x 1000) 10,000 sharks. If the BRD has lowered the catch rate by say
20%, then the mean daily catch rate is now 8 sharks per day of effort and
presuming that effort remains the same, then the annual catch rate for the
fishery becomes 8000 sharks. You should also include standard errors (SEs)
for both the mean incidental catch rate (or mortality rate) and the annual
effort levels. There are formulae for multiplying two means with SEs in
order to derive final total SEs. If you don't want to use effort in the
calculation of final incidental catch/mortality, you can use catch of the
targeted species instead. So for example, if you know that 10 sharks are
caught incidentally for every 1 tonne of targeted catch reported and that
logbook data indicate the mean annual catch is 1000 tonnes, then your
estimate of incidental shark catch is (10 sharks x 1000) 10,000.
Hope this helps.
----- Original Message -----
From: "Kate LeFeaux" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Saturday, December 20, 2008 4:53 AM
Subject: Impact of a bycatch reduction device
> I have a question and would value your thoughts to how best address this.
> What do you think would be the best way to demonstrate the positive impact
> of a bycatch reduction device on local populations (of sharks). Have CPUE
> and length data. BRD significantly reduces CPUE for some species but
> wonder if there is a way to show the benefit (if any) to the population in
> a numerical sence.
> I have some ideas and would value any of your opinions.
> K. LeFeaux