Christofer H. Boggs

Prey selection naticid gastropods; experimental tests and application to the fossil record

Because predation by drilling gastropods is uniquely preservable in the fossil record, it represents important evidence for the study of coevolution. Previous studies of drilling gastropod predation have been largely descriptive and sometimes contradictory. We formulate and test a model of prey selection by naticid drilling gastropods. The model adequately predicts both prey species selection and prey size selection. Prey preferences parallel prey profitabilities, determined by calculating prey species-specific and predator size-specific cost-benefit functions. The model also specifically suggests the evolution of potential refugia from predation and the evolution of potential predatory attributes. Application of the model to several Miocene and Pliocene assemblages studied by Thomas (1976) corroborates the feasibility and utility of this approach in examining the evolutionary record of naticid predation, which extends from the Late Mesozoic. Apparent evolutionary stasis and convergent morphological trends among prey species may be consistent with continuous selection pressures against predation.

The Role of Sharks and Longline Fisheries in a Pelagic Ecosystem of the Central Pacific

The increased exploitation of pelagic sharks by longline fisheries raised questions about changes in the food webs that include sharks as apex predators. We used a version of Ecopath/Ecosim models to evaluate changes in trophic interactions due to shark exploitation in the Central North Pacific. Fisheries targeted on blue sharks tend to produce compensatory responses that favor other shark species and billfishes, but they have only modest effects on the majority of food web components. Modest levels of intraguild predation (adult sharks that eat juvenile sharks) produce strong, nonlinear responses in shark populations. In general, analysis of the Central North Pacific model reveals that sharks are not keystone predators, but that increases in longline fisheries can have profound effects on the food webs that support sharks.

Ability of Archival Tags to Provide Estimates of Geographical Position Based on Light Intensity

We tested the ability of archival tags and their associated algorithms to estimate geographical position based on ambient light intensity by attaching six tags (three tags each from Northwest Marine Technologies [NMT] and Wildlife Computers [WC]) at different depths to a stationary mooring line in the Pacific Ocean (approx. 166°42′W, 24°00′N), for approximately one year (29-Aug-98 to 16-Aug-99). Upon retrieval, one tag each from the two vendors had malfunctioned: from these no data (NMT) or only partial data (WC) could be downloaded. An algorithm onboard the NMT tag automatically calculated geographical positions. For the WC tags, three different algorithms were used to estimate geographical positions from the recorded light intensity data. Estimates of longitude from all tags were significantly less variable than those for latitude. The mean absolute error for longitude estimates from the NMT tags ranged from 0.29 to 0.35°, and for the WC tags from 0.13 to 0.25°. The mean absolute error in latitude estimates from the NMT tags ranged from 1.5 to 5.5°, and for the WC tags from 0.78 to 3.50°. Ambient weather conditions and water clarity will obviously introduce errors into any geoposition algorithm based on light intensity. We show that by applying objective criteria to light level data, outliers can be removed and the variability of geographical position estimates reduced. We conclude that, although archival tags are suitable for questions of ocean basin-scale movements, they are not well suited for studies of daily fine scale movement patterns because of the likely magnitude of position estimate errors. For studies of fine scale movements in relation to specific oceanographic conditions, forage densities and distance scales of 100 km or less, other methods (e.g. acoustic tracking) remain the tool of choice.

Cluster analysis of longline sets and fishing strategies within the Hawaii-based fishery

In the Hawaii-based longline fishery, changes in fishing operations to target different species produce changes in the effectiveness of fishing effort units. Catch-per-unit-effort (CPUE) indices used in resource monitoring were improved by segregating dissimilar types of fishing effort. Cluster analysis was used to classify longline sets in relation to species composition of the catches. Based on proportions of eight species and three broader species groups in 46 961 longline sets from 4 years (1991-1994) of commercial fishery data, five effort clusters were identified. Spatial distribution of sets and differences in fishing operations among clusters were then compared to reveal apparent differences in fishing strategies. Three clusters comprised N 80% of the total sets, and the catch compositions suggested targeting for either broadbill swordfish (two clusters) or bigeye tuna. The other two clusters were most similar to the tuna cluster, but their catch compositions indicated a mixed-species fishing strategy. Fishing operations were most different between sets in the tuna and swordfish clusters. Swordfish sets were characterized by (1) the largest vessels, (2) the least number of hooks per set, (3) the greatest number of lightsticks, (4) the longest set duration, (5) the highest percentage of night sets, (6) a larger percentage of sets within the full moon phase, and (7) the lowest percentage of sets within the main Hawaiian Islands Exclusive Economic Zone. Time series of CPUE for three species (bigeye tuna, yellowfin tuna, and swordfish) based on different clusters were compared, and the most appropriate CPUE time series for resource monitoring are recommended. 0 1997 Elsevier Science B.V.

Tuna Metabolic Rates Estimated from Energy Losses during Starvation

Metabolic rates were calculated from energy losses over time in fasting, freeswimming tuna (30-50 cm fork length). Water content accurately predicted caloric density measured by bomb calorimetry. A subsample that accurately predicted water and energy content in whole tuna was found. The mean metabolic rate for Katsuwonus pelamis starved < 1 wk was 0.36 kJ·d⁻¹ · g⁻¹ wet mass. Tuna kept for longer periods had lower and less variable swimming speeds and metabolic rates. Mean rates for 1-2 wk of starvation were 0.22 kJ·d⁻¹· g⁻¹ for K. pelamis and 0.18 kJ·d⁻¹ · g⁻¹ for both Euthynnus affinis and Thunnus albacares. For >2 wk of starvation, the mean rates were 0.14 kJ·d⁻¹· g⁻¹ for K. pelamis and 0.12 kJ·d⁻¹ · g⁻¹ for both E. affinis and T. albacares. Our energy-loss estimates of metabolic rate covered the same range as K. pelamis respiration rates. Energy losses in fasting tuna were about twice as high as in fasting cold-bodied fish under equivalent conditions.