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Observer Abdoulaye Djiba
Observer Abdoulaye Djiba from Senegal's IFAN institute keeping watch for marine mammals
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5th Cruise Diary: Keeping a watchful eye on the horizon - Visual observations of seabirds and marine mammals

The length of the Nansen cruise between Dakar and Agadir is the second for the CCLME project to include dedicated bird observations, following the June cruise around Cape Verde. Whilst the numbers of breeding and wintering waterbirds at key sites along the coast of the CCLME are reasonably well known, there is very little information on birds out of sight of land. A few pioneering gps and satellite tagging projects are beginning to show how several species of shearwaters use the CCLME.

However, there is no information on offshore hotspots and whether, as in many other marine ecosystems, distributions have shifted to become more dependant fishing industry activity. Of special interest, much of the global populations, of Mediterranean breeding origin, of Audouin's gull Larus audouinii and “Scopoli's” Cory's shearwater Calonectris diomedea diomedea winter in CCLME.

The combination of a good coverage of the continental shelf with transects and regular trawls is providing unique data, through continuous visual observation and the first exploratory analysis. The large numbers of birds attracted to trawls are beginning to elucidate what species are attracted to discards, though there is much on-going analysis to tease out other correlations. In contrast, the closely related Cape Verde shearwater Calonectris edwardsii, the CCLME's one endemic seabird species, showed no interest in the Nansen's trawls during the Cape Verde cruise, within the species' breeding period.

A side interest of the present cruise has been observations of colour ringed birds landing on the vessel, which can be individually identified from the ring colour and code. We have so far had two Scottish Sandwich terns Sterna sandvicensis and three Spanish Audouin's gulls.

Another line of research newly added to the CCMLE programme since October 2011 is an explorative visual survey of marine mammals in the study area. With tourists world-wide enjoying whale- and dolphin watching, searching for cetaceans would appear like the simplest task conceivable. Not quite. In a scientific context, the search mode requires scanning 180° of sea ahead of the ship more or less continuously with 7x50 binoculars for even the slightest cue of cetaceans, such as a splash, a blow or a dorsal fin, up to 10 kilometers away.
This effort must be sustained consistently for 8-9 hours per day, for 4-5 weeks, in some surveys even 2-3 months. Off-effort days, allowing some rest, due to adverse weather may be rare (zero during this survey). Despite the intense search effort, in areas of low density sightings may be spaced far apart, many hours or even a whole day. A tricky situation, as observers may lose concentration and start missing cues, resulting in an even lower sighting probability. To keep up a constant search mode, an enormous amount of discipline and experience is essential.

Another challenge results from the fact that marine mammals were not originally considered part of the CCMLE programme, hence are presently low priority. With no dedicated ship time for closing, cetaceans must be positively identified in “passing mode”, without approaching them. Needless to say, even for an expert, for sightings at several km distance, definite identification is often not feasible, and many records remain registered as generic “delphinids” or “large whale”.

As soon a sighting is made, a number of parameters must be registered before these change or are forgotten (e.g. time, position, estimated distance, heading) or before the mammals disappear from view. Simultaneously ofcourse they must be carefully observed, identified and photographed. The animal(s) are typically in view less than a minute, a few surfacings of seconds each. More often than not the cetaceans move fast and expose frustratingly little of their body. Despite a 300mm lens, the large majority of photographed animals appear as tiny dark dots on a vast expanse of water, nothing comparable to the magnificent photos obtainable during whale-watching trips when the vessel's purpose is approaching the closest possible.

So far seven species, five dolphin species (odontocetes) and two baleen whales (mysticetes) have been observed. In the past, West African waters have hardly been surveyed for marine mammals and most of what we know is derived from strandings or by-catches. Therefore these sightings, despite their shortcomings, once analysed and related to other information and literature are expected to yield some useful insights. Admittedly, new explorative research typically generates many more new questions than answers.

Except for the common dolphins, which were attracted, a surprisingly low fraction of dolphins approached the ship to bowride, indeed several dolphin groups visibly changed course to avoid us. Why is unknown, as our vessel is harmless, but learned avoidance behaviour towards other similarly-sized vessels, possibly including (noisy) seismic survey vessels and industrial fishing vessels which may cause accidental captures, could have something to do with this. However, this is guesswork and we may never know the answer.