Could the Tristan powerboat CPUE decline have been caused by overfishing?

From 2006 to 2014 the CPUE from the powerboats at Tristan has dropped by about 75%, suggesting that exploitable biomass fell by a similar proportion.

The Tristan assessment (Johnston and Butterworth 2015) interprets this primarily as the downside of a decline as a result of the very strong 1997 year class passing through and then out of the fishery, together with some poor recruitment over the first decade of the 21st century. This strong year-class raised the resource well above the average abundance to be expected in the absence of any fishing (the “carrying capacity”); although the assessment indicates that this abundance is now below carrying capacity, the extent by which it is below suggests that resource status is not nearly as poor as that some 75% fall suggests.

An understandable concern does, however, remain that the dominant reason for this decline could still be overfishing. To examine this possibility, we apply a simple model which makes that assumption, to check whether plausible results eventuate and so indicate that this is indeed a viable alternative interpretation.