Incorporating hypoxia-based habitat compression impacts into the stock assessment process for tropical pelagic billfish and tuna

Principal Investigator: Eric Prince
Co-Principal Investigators: Heidi Dewar (SW)
External Collaborator: John Hoolihan (University of Miami), Jiangang Luo (University of Miami), David Die (University of Miami), Mark Maunder (IATTC), and Phillip Goodyear (Independent Scientist)

Large areas of cold hypoxic water, known as Oxygen Minimum Zones (OMZs), occur as permanent features in the Eastern tropical Pacific and Eastern tropical Atlantic Oceans. This layer is defined by a shallow thermocline at a depth around 25-100 m, and the cold hypoxic environment below the thermocline acts as a lower habitat boundary for billfishes and most tunas. As a result, their habitat is compressed and restricted to the shallow mixed layer making them more vulnerable to exploitation by surface fishing gears. Other species (e.g. bigeye tuna and swordfish) are more tolerant of hypoxia, and show diel migrations as they follow the deep scattering layer between the mixed layer at night and deeper waters below the thermocline during the day. However, even these species are still impacted by the habitat compression in the surface mixed layer and the vast densities of prey that reside there.

The Atlantic OMZ has expanded over the past 5 decades by about 15%, further compressing the surface layer and progressively increasing the density of billfishes and tunas, as well as their preferred prey, into shallower surface areas of the Eastern tropical Atlantic. This increase in density of the fish community - including both predators and prey - is likely to lead to an increase in catchability that may bias estimates of indices of abundance derived from catch per unit of effort data. Up to this point, neither large scale environmental influence of the OMZs on habitat use and distribution nor the associated changes in fishing patterns have been incorporated into assessment models or relative abundance estimation in any meaningful way. Hence, considering the obvious differences in catchability inside and outside the compression areas and the possibility that these areas are changing in spatial distribution, it is essential to develop a method to incorporate information on habitat related to large scale environmental features, such as the OMZ, in the stock assessment process.

The objective of this proposal is to take the first step towards developing methods/approaches to incorporate the large scale environmental features of the OMZ and associated habitat compression consequences, into the stock assessment process by 1) applying appropriate statistical models that incorporate data on these features in the estimation of relative abundance from catch per unit of effort data and 2) developing population models that contain enough spatial structure as to represent these large scale features (i.e. OMZ size metrics).

FY 2015 Project 15-025   FY 2015 Project 15-025

Figure 1. a) Distribution of bigeye tuna catches for the period 1995-2004 (source ICCAT) overlaid on a map of dissolved oxygen levels at 100m depth (color coded, see insert legend); and b) map of average SST for the period 1995-2004. White lines represent the possible population compartments to be used in the stock assessment model.

FY 2015 Project 15-025

Figure 2. Distribution of blue marlin catches during the 2000 decade (1995-2004) overlaid on top of the Atlantic Oxygen Minimum Zone. Dissolved Oxygen levels are color coded (see insert legend).

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Southeast Fisheries Science Center (SEFSC)


Annual Report - Year 1

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