Pelagic species assemblages in Hawaiian waters (from Dec. 2007)
We are currently using fishery data to study the patterns and trends of species assemblages in the Hawaiian waters.
Global chlorophyll trends (Feb. 2007 - Dec. 2007)
We used satellite ocean color data from the SeaWiFS sensor to study long term (9 years) trends in global regions of the ocean. We especially focused on the most oligotrophic (low chlorophyll concentration) regions of each oceanic basin.
The SeaWiFS mission has been providing satellite-based estimates of surface chlorophyll since September 1997. We now have an unprecedented, almost decade-long, time series of global ocean surface chlorophyll.
Subtropical gyres comprise the largest ecosystem in each of the major ocean basins and occupy approximately 40% of the surface of the earth. While the biological productivity of these oligotrophic ecosystems is low, they are sometimes referred to as biological deserts; their large size allows for a significant contribution on the global scale.
Temporal trends in the size of these gyres are indicators of changes in ocean biology and due to their large size even modest changes can result in significant impacts on the spatial distribution of surface chlorophyll in the entire ocean.
The temporal trends in the monthly area of the oligotrophic gyres and mean SST (Sea Surface Temperature) were computed with a Generalized Additive Model (GAM). We fitted the monthly area time series with a model that contains a linear function of time to model the time trend and a smoothed monthly term to model the seasonal pattern.
We found that, since 1998, the least productive oceanic habitats in four of the world’s major oceans (North Pacific, South Pacific, North Atlantic, and South Atlantic) have been expanding at average rates between 0.8%/yr and 4.3%/yr.
Mean monthly SST over the subtropical gyres is increasing concurrently with the expansion of the oligotrophic gyres which is consistent with the hypothesis that as the subtropical gyres become warmer, they become more stratified and the oligotrophic gyres expand.
Reference
Polovina, J.J., Howell, E.A., Abecassis, M., 2008, Ocean’s least productive waters are expanding. Geophysical Research Letters, 35, L03618, doi:10.1029/2007GL031745.
Influence of feeding on stingrays in the lagoon of Moorea
(Nov. 2006 - Jan. 2007)
14 stingrays (Himantura Fai) were tagged with passive acoustic tags
between March and July 2005, and 6 multi-directional hydrophones (acoustic receivers) were deployed
in the lagoon of Moorea (French Polynesia), between the Fareone and Tiahura motus and the Papetoai channel.
The objective of this (on going) project is to study ray movements inside the lagoon, and assess the impact commercial feeding activities have on their behavior, since two feeding sites were created in that area in 1995 and 1999, which attract several dozens, and sometimes hundreds of tourists a day.
This North-western part of the lagoon was made a marine protected area (Tiahura marine reserve) in 2004, and this study was funded by the government of French Polynesia and by the French Secretary of Overseas Territories. It is led by the non-profit organization Te Mana O Te Moana, directed by Cécile Gaspar, in collaboration with the CRIOBE.
Between Nov. 2006 et Jan. 2007, I analized presence/absence data recorded by the receivers to study the activity of rays around feeding sites and elsewhere in the lagoon.
8 other rays (4 fed ones and 4 wild ones) were also tagged with active acoustic tags and tracked with a small boat during a few hours to 3 days.
I used Arcview 3 and the Animal Movement extension developed by Philip Hooge from USGS, Alaska, to determine the home range of the rays which exhibited some site fidelity, in order to study the differences between fed and wild rays.
Vertical movements of opah (2006)
Opah (Lampris guttatus) is a mid-water pelagic fish incidentally
caught by Hawaii longline vessels targeting bigeye tuna (Thunnus obesus).
Particularly valued by restaurants, this exotic fish is generally harvested in small, but
nevertheless significant quantities. Since it is not a targeted species, this fish has historically been poorly studied and as a result, available information pertaining to the biology and ecology of this resource are virtually nonexistent.
This study focused on vertical movements of opah, investigated by tagging fishes (n=10) with pop-up satellite archival transmitting tags (PSATS) recording depth, temperature and light.
We tried to describe the characteristics of the vertical behavior of opah, and relate it to oceanographic features in the North Pacific, using remote sensed alimetry data from the AVISO centre and oceanic data from the World Ocean Atlas.
We found that, in the subtropical gyre northwest of the Hawaiian Islands, opah generally inhabited a 50–400 m depth range and 8–22ºC temperatures. They were frequently found deeper (100–400 m) during the day than at night (50-150m) but were constantly moving vertically within this broad range.
Their vertical speeds were generally < 25 cm/s but on one occasion an opah descended at a burst speed of 4 m/s, which may have been a predator-avoidance response.
Vertical habitat use by individual opah apparently varied with local oceanographic conditions, but over a 24-h period, the average temperature experienced was always in the narrow range of 14.7 to 16.5ºC.
To estimate the tracks followed by the fish we tagged, we used the excellent R Trackit package developed by Anders Nielsen and John Sibert from the Pelagic Fisheries Research Program (PFRP), SOEST, Hawaii. This package provides a geolocation estimation based on the light levels recorded by the tag (two measurements per day).
An examination of Aviso sea surface height (SSH) data, showed one of the tags encountered a cyclonic eddy. Unfortunately, the 95% confidence intervals on the daily position estimates were too large to determine if the opah traveled through the center of the eddy or around the edge.
However, the temperature–depth structure constructed from the opah vertical movement showed that during the same period, the opah occupied a region with a broad temperature–depth relationship. This temperature–depth relationship suggests the opah was in the convergent or downwelling edge of the eddy rather than the compact temperature–depth relationship that would be expected if the opah were in the divergent or upwelling center of the eddy.
Reference
Polovina, J.J., Hawn, D., Abecassis, M., 2008, Vertical movement and habitat of opah (Lampris guttatus) in the central North Pacific recorded with pop-up archival tags. Marine Biology, 153:257–267.
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