Showing papers by "Molly Lutcavage published in 2015"

The paradox of the pelagics: why bluefin tuna can go hungry in a sea of plenty

Abstract: Large marine predators such as tunas and sharks play an important role in structuringmarine food webs. Their future populations depend on the environmental conditions they en -counter across life history stages and the level of human exploitation. Standard predator−prey rela-tionships suggest favorable conditions (high prey abundance) should result in successful foragingand reproductive output. Here, we demonstrate that these assumptions are not invariably validacross species, and that somatic condition of Atlantic bluefin tuna Thunnus thynnus in the Gulf ofMaine declined in the presence of high prey abundance. We show that the paradox of decliningbluefin tuna condition during a period of high prey abundance is explained by a change in the sizestructure of their prey. Specifically, we identified strong correlations between bluefin tuna bodycondition, the relative abundance of large Atlantic herring Clupea harengus , and the energeticpayoff resulting from consuming different sizes of herring. This correlation is consistent withoptimal foraging theory, explaining why bluefin tuna condition suffers even when prey is abundant.Furthermore, optimal foraging principles explain a shift in traditional bluefin tuna foraging areas,toward regions with a higher proportion of large herring. Bluefin tuna appear sensitive to changesin the size spectrum of prey rather than prey abundance, impacting their distribution, reproductionand economic value. Fisheries managers will now face the challenge of how to manage for highabundance of small pelagic fish, which benefits benthic fishes and mammalian predators, andmaintain a robust size structure beneficial for top predators with alternative foraging strategies.KEY WORDS: Bluefin tuna · Herring · Optimal foraging · Condition ·

Diet and condition of Atlantic bluefin tuna ( Thunnus thynnus ) in the Gulf of Maine, 2004–2008

Abstract: Atlantic herring (Clupea harengus) are the primary forage for Atlantic bluefin tuna (Thunnus thynnus; ABFT) in the Gulf of Maine. Recent studies have documented significant declines in ABFT somatic condition and shifts in their size and spatial distribution in the Gulf of Maine, which may be linked to trophic changes. We collected stomachs (n = 122) as well as liver (n = 110) and white muscle (n = 382) samples for carbon and nitrogen stable isotope analysis and lipid analysis to determine if diet composition had changed relative to the late 1980’s to early 2000’s for large, commercially harvested ABFT (≥185 cm curved fork length (CFL)). Samples of smaller ABFT (<185 cm CFL) were also collected (stomachs: n = 21; liver: n = 17; white muscle: n = 19) to compare diet between size classes. Large ABFT diet was similar among current and historic studies, Atlantic herring being the main prey (39.5 to 52.8 % weight). Small ABFT fed at a lower trophic position (TP = 3.9) than larger individuals (TP = 4.9) due to higher consumption of sand lance (Ammodytes spp.) and euphausiids (65.6 vs. 4.5 % weight). Mean and maximum lipid stores of large ABFT increased from spring through fall, but lean fish were observed in all seasons. In the fall, lean ABFT were lighter with lower nitrogen and higher carbon isotope values than co-occurring ABFT with higher lipid stores. These patterns are consistent with shelf vs. offshore isotope baseline differences in the western North Atlantic and variable arrival and residency patterns for ABFT in the Gulf of Maine.

Sexual maturity in western Atlantic bluefin tuna

Abstract: We introduce a novel endocrine approach for assessing the unresolved matter of the timing of sexual maturation in western Atlantic bluefin tuna (ABFT), a highly migratory population whose status remains uncertain. Ratios of follicle stimulating hormone to luteinizing hormone, a sexual maturity indicator, in all ABFT ≥ 134 cm curved fork length (CFL) were <0.4, similar to Mediterranean spawners, indicating that western ABFT mature at considerably smaller sizes and at a much younger age than currently assumed (≥ 185 cm CFL).

Atlantic bluefin tuna Thunnus thynnus feeding ecology in the northern Gulf of Mexico: a preliminary description of diet from the western Atlantic spawning grounds

Abstract: A combination of stomach contents, nitrogen stable-isotope and tissue C:N values are presented to demonstrate feeding activity of Atlantic bluefin tuna Thunnus thynnus on the Gulf of Mexico (GOMEX) spawning grounds. Diets include teleosts, cephalopods, crustaceans and a pelagic tunicate (Pyrosoma atlanticum). Results reveal the need to classify the GOMEX as a T. thynnus feeding ground.

Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre.

Abstract: Leatherback sea turtles (Dermochelys coriacea) travel thousands of kilometres between temperate feeding and tropical breeding/over-wintering grounds, with adult turtles able to pinpoint specific nesting beaches after multi-year absences. Their extensive migrations often occur in oceanic habitat where limited known sensory information is available to aid in orientation. Here, we examined the migratory orientation of adult male, adult female and subadult leatherbacks during their open-ocean movements within the North Atlantic subtropical gyre by analysing satellite-derived tracks from fifteen individuals over a 2-year period. To determine the turtles' true headings, we corrected the reconstructed tracks for current drift and found negligible differences between current-corrected and observed tracks within the gyre. Individual leatherback headings were remarkably consistent throughout the subtropical gyre, with turtles significantly oriented to the south-southeast. Adult leatherbacks of both sexes maintained similar mean headings and showed greater orientation precision overall. The consistent headings maintained by adult and subadult leatherbacks within the gyre suggest use of a common compass sense.