1.2 History of tag based geolocation
Tagging of sh is a wide spread technique to gain information of behaviour and to obtain global positional estimates of the tagged individual.
1.2.1 Conventional tags
A tagging experiment consists of mounting simple markers on sh in a way that has the least possible eect on the behaviour and growth (Righton et al., 2006).
A batch of sh is released into the sea with the intention that some percentage is recaptured and their tag recovered. This type of mark/recapture experiments, or conventional tagging, were initially a mean to asses the mortality of sh by evaluating the return rate of the tags. As a side product, the experiments also supplied information of the recapture positions that gave rise to tag based geolocation.
Conventional tagging methods yield only a sparse dataset per returned tag, and therefore requires extensive tagging for major conclusions on the distribu-tion of individuals to be made. Fortunately the procedure is associated with low costs and has been carried out since the mid sixties up until the present day, hence a substantial amount of data is available (Daan, 1978;Righton et al., 2007). However, the number of returns from a given geographical area is largely inuenced by the shing eort, thus diminishing the statistical power of the data.
The present thesis focuses on the Atlantic cod henceforth referred to as cod -and the habitats of the North Sea -and the English Channel. Figure 1.2shows a map of the ICES areas that are contained in the considered domain. Previ-ous work has shown that cod released in the southern North Sea tend to either stay in a limited area close to the release position or migrate north (Righton et al., 2007). Migration is often performed in an annual cycle bringing the cod to the central part of the North Sea (ICES IVb) in the summer, before returning south during the winter (Righton et al., 2007). This behaviour is conrmed by research based on DTSs (Righton et al., 2000). No annual migration cycle has so far been proven by conventional tagging for cod released in the English Channel. In fact, not much can be said about cod released in VIId besides that the majority was recaptured close to the release location regardless of its time at liberty (Righton et al., 2000; Righton et al., 2007).
The obvious drawback of conventional tagging is the scarce amount of data returned from one tag, rendering it dicult to deduce the behaviour whilst at
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Figure 1.2: Map showing the ICES areas.
liberty. A cod recaptured close to its release position could possibly have made large excursions in the intervening period. It was therefore a great advance for the eld of geolocation when DSTs where introduced as data collectors.
1.2.2 Data Storage Tags
DSTs come in a variety of types and sizes (see Section 5.2) and have in their short history been used for geolocation of many kinds of marine animals. For
1.2 History of tag based geolocation 5 cod, the tagging procedure itself has developed as well, to cover both external and internal tagging of the sh (Righton et al., 2006). Compared to conventional mark/recapture tagging, the DST experiments have substantial added costs. It is therefore of great interest to extract maximal information from a successfully returned tag.
In the tagging procedure emphasis is put on minimising the traumatisation of the individual. The cod is either caught by line or by trawl and brought to the surface slowly to avoid swimbladder rupture. Here they are anaesthetised before the tag is mounted, either externally next to the rst dorsal n, or in-ternally in the peritoneal cavity along with an external marker (Righton et al., 2006).
When the sh is released into the sea the DST logs information of the envi-ronment such as depth, light, temperature or salinity. The choice of measure depends in general on the species and its immediate environment. For example in the Baltic Sea, tagging experiments have been performed mostly with DSTs measuring depth, temperature and salinity exploiting the, in some areas, large gradients of these quantities (Neuenfeldt et al., 2006).
In the Pacic Ocean for tracking bigeye tuna, DSTs measuring ambient light have been used. The uncertainty of the light based geolocation is very seasonal dependent and increases especially around the equinox (Musyl et al., 2001).
Another type of DST used for geolocation is a pop-up satellite archival tag (PSAT). The tag self-releases from the animal at a preprogrammed time and transmits the data via satellite when reaching the surface. Due to the transmis-sion process the PSAT has a large battery requirement compared to a DST and the amount of retrievable data is in general limited.
PSATs are normally used for animals that are not targeted by commercial sh-ermen, and therefore satellite transmission is the only way of retrieving the data. Among the applications are investigations of the dive behaviour and post-release mortality following interactions with longline shing gear of olive ridley sea turtles (Lepidochelys olivacea) (Swimmer et al., 2006), and geolocation of Greenland sharks (Somniosus microcephalus) (Stokesbury et al., 2005).
Pressure measurements from demersal species have a great potential for geoloca-tion. When the sh dwells at the sea bed for a longer period of time, the pressure recorded by the DST is constant except for variations following the tide. This tidal signal is compared to a numeric tidal forecast system and the possible posi-tions can be found. A greater study using tidal patterns for geolocation was con-ducted successfully on plaice (Pleuronectes platessa L.) in the North Sea (Hunter et al., 2004). Tidal location work in progress focus also on other demersal species
such as sole (Solea solea) and ray (Raja clavata), aiming to clarify migration routes and seasonal behaviour etc. Likewise, the Atlantic cod has been subject to ongoing DST research of which some results are presented inTurner et al. (2002);
Righton et al. (2007).