Kerim Aydin

Visualizing the Food-Web Effects of Fishing for Tunas in the Pacific Ocean

We use food-web models to develop visualizations to compare and evaluate the interactions of tuna fisheries with their supporting food webs in the eastern tropical Pacific (ETP) and the central north Pacific (CNP) Oceans. In the ETP and CNP models, individual fisheries use slightly different food webs that are defined by the assemblage of targeted tuna species. Distinct energy pathways are required to support different tuna species and, consequently, the specific fisheries that target different tuna assemblages. These simulations suggest that catches of tunas, sharks, and billfishes have lowered the biomass of the upper trophic levels in both systems, whereas increases in intermediate and lower trophic level animals have accompanied the decline of top predators. Trade- offs between fishing and predation mortality rates that occur when multiple fisheries interact with their respective food webs may lead to smaller changes in biomass than if only the effect of a single fishery is considered. Historical simulations and hypothetical management scenarios further demonstrate that the effects of longline and purse seine fisheries have been strongest in upper trophic levels, but that lower trophic levels may respond more strongly to purse-seine fisheries. The apex predator guild has responded most strongly to longlining. Simulations of alternative management strategies that attempt to rebuild shark and billfish populations in each ecosystem reveal that (1) changes in longlining more effectively recover top predator populations than do changes in purse seining and (2) restrictions on both shallow-set longline gear and shark finning may do more to recover top predators than do simple reductions in fishing effort.

BOUNDARIES OF OPEN MARINE ECOSYSTEMS: AN APPLICATION TO THE PRIBILOF ARCHIPELAGO, SOUTHEAST BERING SEA

We applied ecosystem energetics and foraging theory to characterize the spatial extent of the Pribilof Archipelago ecosystem, located in the southeast Bering Sea. From an energetic perspective, an ecosystem is an area within which the predatory demand is in balance with the prey production. From a foraging perspective, an ecosystem boundary should at least include the foraging range of the species that live within it for a portion of their life cycle. The Pribilof Islands are densely populated by species that adopt a central place foraging strategy. Foraging theory predicts that the area traveled by central place foragers (CPF) should extend far enough so that their predatory demands are in balance with prey production. Thus, the spatial extent of an ecosystem, as defined by energetics and the foraging range of constituent species, will require a similar energy balance, and independent assessments should yield similar results. In this study, we compared the area of maximum energy balance, estimated with a food web model during the decade 1990- 2000, with estimates of the foraging range of northern fur seals (the farthest traveling CPF in the Pribilof Islands community) obtained from the literature. From the food web sim- ulations, we estimated that a circle of 100 nautical miles (NM), or 185.2 km, radius encloses the area of highest energy balance and lowest biomass import and that it represents a switch from a piscivorous-dominated (smaller areas) to a zooplanktivorous-dominated (larger ar- eas) community. The distance from the breeding site to locations recorded at sea for lactating female fur seals, during the years 1995-1996, ranged from 5.0 to 172.2 NM (9.3-318.9 km), with a median of 97 NM (179.6 km). Thus, ;50% of the locations recorded for lactating fur seals occurred beyond the area of energy balance estimated by the model, indicating that additional factors can motivate their foraging extent. We propose that en- ergetic constraints set the minimum extent of the Pribilof ecosystem, while the foraging distance of fur seals may indicate the maximum extent. In discussing these results, we highlight the limitations of current definitions of the spatial extent in ecosystems, when related to open oceanic environments, and discuss viable alternatives to characterize bound- aries of aquatic systems that are not physically separated from adjacent areas. We believe that these arguments, though controversial, are very timely given the increased emphasis currently placed on the management and protection of entire marine ecosystems.

A trophic mass balance model of the eastern Chukchi Sea with comparisons to other high-latitude systems

Abstract The Chukchi Sea is a seasonally ice-covered, marginal Arctic-shelf sea that possesses both large petroleum reserves and abundant biological communities, including migratory mammals and seabirds. We developed a mass balance food web model for the eastern Chukchi Sea to evaluate the trophic structure of this ecosystem and to compare food web properties of the Chukchi Sea to those of other high-latitude marine ecosystems. We compiled data on biomass levels, diet composition, demographic rates (production, consumption), and fishery removals, and used these data to construct an Ecopath trophic mass balance model. The majority of biomass was concentrated in benthic invertebrates and most of the mass flow above trophic level 2.0 was through these groups. We found that density estimates of most fish groups derived from trawl survey data using area-swept methods were insufficient to match the consumptive demands of predators, and that densities needed to be several-fold greater to meet modeled demand. We also used a set of system metrics derived from a common modeling framework to highlight differences in ecosystem structure between the eastern Chukchi Sea and other high-latitude systems. The extent of benthic dominance observed in the eastern Chukchi Sea was unique among the systems examined, both in terms of food web structure and associated mass flows between benthic and pelagic components. In relation to total biomass density, the eastern Chukchi Sea had low production when compared with the other systems, and this lower turnover rate suggests that recovery from disturbance might be slow.

Ecosystem considerations in Alaska: the value of qualitative assessments

&NA; The application of ecosystem considerations, and in particular ecosystem report cards, in federal groundfish fisheries management in Alaska can be described as an ecosystem approach to fisheries management (EAFM). Ecosystem information is provided to managers to establish an ecosystem context within which deliberations of fisheries quota occur. Our goal is to make the case for the need for qualitative ecosystem assessments in EAFM, specifically that qualitative synthesis has advantages worthy to keep a permanent place at the fisheries management table. These advantages include flexibility and speed in responding to and synthesizing new information from a variety of sources. First, we use the development of indicator‐based ecosystem report cards as an example of adapting ecosystem information to management needs. Second, we review lessons learned and provide suggestions for best practices for applying EAFM to large and diverse fisheries in multiple marine ecosystems. Adapting ecosystem indicator information to better suit the needs of fisheries managers resulted in succinct report cards that summarize ecosystem trends, complementing more detailed ecosystem information to provide context for EAFM. There were several lessons learned in the process of developing the ecosystem report cards. The selection of indicators for each region was influenced by geography, the extent of scientific knowledge/data, and the particular expertise of the selection teams. Optimizing the opportunity to qualitatively incorporate ecosystem information into management decisions requires a good understanding of the management system in question. We found that frequent dialogue with managers and other stakeholders leads to adaptive products. We believe that there will always be a need for qualitative ecosystem assessment because it allows for rapid incorporation of new ideas and data and unexpected events. As we build modelling and predictive capacity, we will still need qualitative synthesis to capture events outside the bounds of current models and to detect impacts of the unexpected.

The role of flatfishes in the organization and structure of the eastern Bering Sea ecosystem

We evaluated the role of flatfishes in the organization and structure of the eastern Bering Sea ecosystem using the Ecopath/Ecosim approach. As basic input data for the Ecopath/Ecosim model, we used estimates of biomass from bottom trawl surveys and age-structured population models, production/biomass (P/B) ratio, consumption/biomass (Q/B) ratio, diet composition (DC), and fisheries harvests for each component of species or species groups. We estimated the trophic level of each component, niche overlaps among flatfishes, and the impacts of competition and predation on flatfish species in the eastern Bering Sea ecosystem. Based on those estimates, we developed the tropho-dynamic structure of the ecosystem, and the model was used to simulate ecological effects of fishery exploitation patterns. No single flatfish species appeared to have a profound and uniquely important role in the organization and structure of the ecosystem. Instead, the most important component among the guild of flatfish species appeared to be yellowfin sole Pleuronectes asper, which had greater biomass than other flatfish and a relatively diverse diet among the small flatfish species. Pacific halibut Hippoglossus stenolepis, Greenland turbot Reinhardtius hippoglossoides, and arrowtooth flounder Atheresthes stomias were important keystone predators in the eastern Bering Sea ecosystem together with some groups of marine mammals and sea birds. Intra flatfish complex cannibalism was not observed, however, substantial diet overlaps were common in the flatfish guild system.

Trophic structure of the eastern Chukchi Sea : an updated mass balance food web model

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Erratum to: The role of flatfishes in the organization and structure of the eastern Bering Sea ecosystem

In the 5th paragraph of the ‘‘Model structure’’ subheading of the ‘‘Data and methods’’, lines 6–8 of the text should read: ‘‘...the immigration of i, Yi is the yield of i (i.e., its catch in weight, with Yi = FiBi, where F is the fishing mortality rate), Bj is the biomass of the consumers or predators,...’’. On the 2nd and 4th pages of Table 2, ‘‘35 Myc. & Bathy.’’ in the column headed ‘‘Prey/predator’’ should be ‘‘35 Myc. & bathy.’’ In Fig. 4, ‘‘Myc. & Bathy.’’ on the y axis should also be ‘‘Myc. & bathy.’’ In addition, under the ‘‘Ecosim analyses’’ subheading of the ‘‘Results’’, in the 1st paragraph lines 9–11 should read: ‘‘...Arrowtooth flounder had biomass 3.7 times higher in 2005, and Greenland turbot showed the fastest rate of decrease’’. In the 2nd paragraph lines 17–18 should read: ‘‘...It was only the rate of increase or decrease in the biomass of flatfishes...’’.