Primary production required (PPR)
\[PPR_t = \sum_{i=1}^{n_t} \left(\frac{landings_{t,i}}{9}\right) \left(\frac{1}{TE}\right)^{TL_i-1}\] where \(n_t\) = number of species in time \(t\), \(landings_{t,i}\) = landings of species \(i\) in time \(t\), \(TL_i\) is the trophic level of species \(i\), \(TE\) = Trophic efficiency. The PPR estimate assumes a 9:1 ratio for the conversion of wet weight to carbon and a constant transfer efficiency per trophic level.
We have explored the index in the following ways. Using:
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A global transfer efficiency of 15% for all species.
This probably needs some refinement and can/should be adapted based on previous studies. One adaptation would be to use a different transfer efficienct for the first level. eg. \(\left( \frac{1}{TE_1}\right) \left(\frac{1}{TE_2}\right)^{TL_i-2}\). Whatever choices are made, the sensitivity of the index to such changes should be examined.
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Primary production not lagged with landings.
This is probably not realistic. You wouldn’t expect to see changes in the landing the same year as changes in primary production. This needs to be explored, either using specific lags in time (which may prove problematic since species lower on the food chain will be effected by shorter lags in time versus species higher up the chain) or by adopting some weighted scheme.
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A threshold of 80% for landings.
It would be a good idea to explore the sensitivity of the index for other threshold levels. Of course the higher the threshold used would imply that less common species will then contribute to the index.
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Combined vertibrates and invertibrates.
The landings in some of the EPUs are dominated by invertibrates (LOBSTER, CLAMS) which may play a significant part in driving this index. Creating two additional indices, one for vertibrates and one for invertibrates may be an interesting avenue. This will of course imply the inclusion of many other lesser caught species into the index. It will also involve partioning the landings into vertibrates and invertibrates.
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Uncertainty Estimates
No uncertainty is presented. This could be included in several ways. And the sensitivity of the index to each of the following examined.
- Trophic level uncertainty - fishbase
- landings uncertainty
- primary production uncertainty
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Survey Comparison
Compare mean trophic level from survey data with commercial landings. (Just for vertibrates). There are some pretty big spikes in MTL, maybe due to invertibrates
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Extend time series
Reacalculate the index going back to the 60’s. Primary production info is not available for that time period however we can make some assumptions (mean, median, max of PP data we do have). The idea is to how we are now relative to the 60’s when we had very large harvests
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Examine composition of species at trophic level
The species composition within a trophic level could change throughout time. This may not be picked up in the index but may be of interest to managers. Need to examine data