(2) Random and directed movement (from the SW)

(3) Random and directed movement (from the NE)

(4) Random and directed movement (from the SE) using centered-space differencing

(5) Random and directed movement (from the SE) using upwind differencing

(6) A seasonal model in which 500 tags are released at the start of each quarter for one year at sites near Kauai, Hawaii and Cross seamount. This model uses the centered-space scheme with open boundary conditions. The directed movement pattern mimics the annual north-south migration of the North Equatorial Current and the random movement pattern is based on observed catch characteristics. (MPEG file 840 kb).

We are concerned with the development of hypotheses for the release regime, natural mortality and movement of tagged yellowfin tuna ("tags") in the Hawaiian Islands. Since the paths taken by released individuals are unknown, the movement parameters that can be determined from tag release-recapture experiments are those of bulk movement, assumed to have both directed and random components. Adopting this viewpoint allows the application of the well known advection-diffusion-mortality model to the release-recapture data, with the advection parameter interpreted as the component of net "directed movement" of the tags and the diffusion parameter as the component of net "random movement"; the mortality parameter will include terms to account for death by both natural and fishing causes.

Abstract of report

Large scale release-recapture programs initiated with the purpose of providing information for fishery management are both lengthy and expensive. The objective of this project is to use computer simulations to establish whether such a program might be useful in answering questions about yellowfin tuna in the Hawaiian Islands EEZ.

Computer simulations were carried out using a seasonal hypothesis for bulk behavioral movement of tagged yellowfin (tags) to determine if the movement parameters could be faithfully reproduced by the available estimation software. Two numerical schemes were compared, each under open and closed boundary conditions. Three values for monthly natural mortality were used. Results for a 10\%/month natural mortality rate are given in detail.

The effect of adjusting reporting rates and numbers of tag releases was examined. Assuming that tag recapture percentages indicate catch percentages, fleet interactions can be estimated by manipulating fleet participation. These experiments, using 1991-92 effort data, suggest that the interaction between longline and nearshore fleets is only a few percent. Interaction between the three nearshore fleets is similarly either negligible or only a few percent, but this latter result may reflect our restructuring of the supplied data set. Restructuring of the original handline data was necessary following input from the National Marine Fisheries Service (NMFS) concerning an offshore group of handliners and was also prompted by otherwise inexplicably high ratios noted at certain locations between observed catch and catch estimates based on global CPUE (catch per unit effort).

Parameter values were accurately recovered in model regions encompassing the release sites. Confidence in the values depends directly on the number of recaptures made in a given region: the greater the number of recaptures, the more accurate the mean of the estimates and the smaller its accompanying standard deviation.

Based on these results and others obtained using simple one-season hypotheses for behavioral movement, we believe that reliable movement and mortality data is recoverable from any sensibly posed tag release-recapture experiment for the Hawaiian Islands EEZ. Release numbers should be of the order of 500 per month. This figure should be achievable using scientific personnel and a dedicated vessel; it is not likely to be achieved using the services and the boats of local fishermen.

Complete text of report

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Links to relevant projects

Dr. Bo Qiu (School of Ocean Earth Sciences and Technology (SOEST), University of Hawaii) (see PFRP project "A numerical investigation of ocean circulation and pelagic fisheries around the Hawaiian Islands") has provided hydrodynamic flowfield data from a two-and-a-half layer model driven by annual climatological winds.

Dr. Pierre Flament (SOEST, University of Hawaii) has provided mean currents and eddy diffusivities for the Hawaiian Islands EEZ based on an analysis of the movement of drifting buoys, which is the focus of his PFRP project "Physical characteristics of the environment influencing pelagic fisheries".

Since the estimation routine is capable of faithfully reproducing the parameters specified in a given hypothesis, some hard data on the actual movement of tagged yellowfin in Hawaiian waters would greatly assist in evaluating specific designs. Such information is being gathered Dr. Kim Holland's PFRP project: "A tag and release program for the Hawaiian seamount yellowfin and bigeye tuna handline and troll fisheries".

Movie of tag movement using SSAP data

The following movie of tuna movement in the Western Pacific, constructed by John Sibert and Richard Bailey, utilizes tag release and return records from the Skipjack Survey Assesment Program (1977-1982), and incorporates logbook effort data for the same period. The upwind differencing scheme was used.

(1) SSAP movement model (MPEG file 2.9Mb).