Modelling in-season pulses of recruitment and hyperstability-hyperdepletion in the Loligo gahi fishery around the Falkland Islands with generalized depletion models

The time-series of daily catch, fishing effort, and mean body mass of the summer fishing seasons of the squid, Loligo gahi, in the Falkland Islands from 1990 to 2009 are investigated with generalizations of depletion models that account for in-season pulses of recruitment (i.e. open populations) and non-linear relationships between catch as the response variable, and effort and abundance as the predictor variables. Two main results were found. First, stock dynamics are more complex than assumed by Leslie–Davis or De Lury depletion models, because in most years, there are several major in-season recruitment pulses, sometimes even larger than the pre-season pulse, contradicting the basic assumption of a decline in catch rates over the fishing season. Second, the fishery operates under a regime of hyperstability—catch rates decline slower than abundance—at low stock abundance and hyperdepletion—catch rates decline faster than abundance—at intermediate and high stock abundance. The hyperdepleted regime is far more prevalent, a result attributed to the availability of refuges from fishing operations, which may lower the abundance threshold to pass from the low-abundance hyperstable regime to the higher-abundance hyperdepleted regime.