2
Strategies and Tactics for the Occupation of Available Territories

2.1. Faithfulness to habitat and birth site

2.1.1. Remaining faithful to one's habitat

2.1.1.1. Almost infallible chemical signatures

Knowledge of the movements of fish in and around their habitat has long been based on "capture-tag-recapture" methods that only involved the testing of a small number of individuals and often short tagging periods. However, these have made it possible to specify the movements of fish within their territory or home range*. As a result, it has been shown that the Australian catfish, Tandanus tandanus, remains in a small area near its deadwood and macrophyte habitat and only exceptionally moves far. The wels catfish, Silurus glanis, also shows great fidelity to its habitat, which stretches over a 12-kilometer stretch of the Meuse in the Netherlands, although it is highly anthropized - boat mooring, power plants - thereby neglecting the possibility of colonizing other sections of rivers or adjacent lakes. Man-made habitats are perfect for this fish. Fidelity to its habitat and a tendency to disperse and explore new habitats were tested in European pike, Esox lucius, using telemetry*, skills that vary according to the seasons (maximum activity in winter and spring) and with individual differences depending on the size and social rank of the individuals; the largest and most dominant are the most adventurous, with an experimental movement or translocation followed by a rapid return, within 6 days, to its original habitat.

The Australian coastal nervous shark, Carcharhinus cautus, shows great fidelity to its mangrove habitat, which provides it with food and protection, although there are strong individual differences in mobility as its home range* can vary from 0.66 km2 to 3.64 km2.

Advances in microchemical techniques that use radioactive isotopes* - strontium 90Sr and manganese 54Mn - have been significant as each individual contains the chemical traces of his or her past and life history. The mineralized items that are most commonly used and most revealing of habitat change are scales and, in particular, otoliths* which serve as a very accurate "black box" (Volume 2, section 3.2). Teeth are less commonly used as markers of life, although these mineralized tissues also carry valuable information. For example, the isotopic composition* of the teeth of the bull shark, Carcharhinus leucas, revealed the existence of strictly marine habitats in Fiji and the absence of freshwater incursions as previously assumed. The composition of the oxygen isotope* d18O2 in the phosphates of the bio-apatite of the dental matrix and the high content, in enamel and dentin, of trace elements of fluorine F, sodium Na, magnesium Mg and selenium Se, combined with the low content of barium Ba, with a high Sr/Ba ratio, accurately and indisputably indicate a marine existence and eliminate the possibility of entry into fresh waters. The large number of teeth of these sharks, their regular loss and permanent replacement favor their use for ecological monitoring purposes. However, the rapid rate of turnover associated with regular tooth replacement does not enable long-term interpretations. Such dental signatures are indicative of a recent lifestyle.

The microchemistry of otoliths can sometimes be misleading, as in the case of the Alaskan Dolly Varden trout, Salvelinus malma, which, feeding heavily on the eggs of returning O. nerka sockeye salmon, have otoliths with Sr/Ca levels typical of those of marine migratory species. Its non-migratory character is confirmed by the Ba/Ca ratio of its otoliths, which is characteristic of species resident in fresh water and reveals the anadromous* life of a fish that does not leave fresh water.

Bibliography: Can.J.Fish.Aquat.Sci, 2015, 72: 422-433 & DOI:10.1139/cjfas-2014-0055, Ecol.Freshwat.Fish, 2015, 24: 443-455 & DOI:10.1111/eff.2015.24.issue-3/issuetoc, Env.Biol.Fish, 2015, DOI:10.1007/s10641-015-0386-4, 2016, 99: 237-247 & DOI:10.1007/s10641-016-0470-4, J.appl.Ichthyol., 2014, 30: 472-478 & DOI:10.1111/jai.2014.30.issue-3/issuetoc, J.Fish Biol, 2015, 87: 323-341 & DOI:10.1111/jfb.2015.87.issue-2/issuetoc

2.1.1.2. A sedentary life in its home range*

Many species lead a more or less sedentary existence and some, referred to as "residents", remain particularly loyal to the habitat they chose in their early years and which they enjoy fiercely defending if a rival tries to dislodge them (Volume 1, section 1.3.2). This preferred habitat offers definite qualities in terms of hydraulic protection against the risks of the movements of the current, anti-predator refuge, sufficient supplies and a breeding area with the presence of potential sexual partners and nesting and laying areas. Fish living on coral reefs are very faithful to their territory, as are the yellow-striped cardinalfish, Ostorhinchus cyanosoma, of the Great Barrier Reef. Monogamous couples show a greater attachment to their habitat than solitary individuals, which is proof that a social life promotes a certain fidelity to its habitat.

Bibliography: Behav.Proc. 2014, 107: 119-126 & DOI:10;1016/j.beproc.2014.07.015

2.1.1.3. Know the area around one's habitat well

Faithfulness to a site requires a perfect knowledge of the topography of the site and its immediate surroundings so that good topographical references can be acquired, ensuring that one never gets lost and is able, in the event of danger, to reach the habitat as quickly as possible.

The possession of spatial memory plays a crucial role in the survival of small species such as guppies, Poecilia reticulata, which are constantly threatened by predation and which must often, as quickly as possible with functional impulsivity, reach secure shelters. Sand gobies of the Pomatoschistus genus learn to locate their microhabitats, whose visual spatial landmarks they have memorized in order to ensure their regular occupation. As a result, Australian species on rocky coasts show a high degree of fidelity to each of their microhabitats, unlike species on sandy beaches, whose spatial occupation is very unstable and more irregular. The damselfish, Stegastes fuscus, can be trained through experiments to learn to recognize a specific site associated with a colorful stimulus that it has memorized. This ability to orientate itself in space using lateral visual and mechanical signals enables it to move successfully through the complex coral reefs that it uses as its habitat. The grouper, Plectropomus leopardus, from Australian reefs, which has been followed using acoustic tracking*, makes various horizontal and vertical movements influenced by temperature, in summer during the day in its home range*, which remains limited to 0.5 km2, as migratory breeding movements appear to be rare. In a marine reserve in New Caledonia, Epinephelus maculatus moves within a dispersed reef habitat with a home range* of approximately 4 km, although it is sometimes able to move more than 7 km in 10 hours. Sedentary lifestyles and exploratory capabilities can be conducive to the recolonization of cyclone-disrupted habitats.

The African mormyrid or elephant fish, Gnathonemus petersii, uses electrical signals (electro-localization) to reach its preferred habitat at night, using the equivalent of an "electrical retina".

Solar orientation - possession of a "solar compass" - is used, on clear days, by mosquitofish, Gambusia, but this ability is lost on cloudy days and at night.

Memory of the sites is vital, especially for intertidal marine species*, such as the blenny Lipophrys pholis on the Atlantic coast, which leaves its habitat at ebb tide to avoid exundation* and which must find it again when it returns at the rising tide, thereby demonstrating a precise memory of these sites, which is all the more important as the habitats are protective shelters that must be preserved. These include family refuges, which are spatially well determined, according to a "spatial map", and towards which it is advisable to orientate oneself using local landmarks. On the Portuguese coast, over three consecutive years of study, this species has shown a high degree of fidelity to its coastal habitat, with nesting males remaining on the same spawning sites they return to each year. If they are moved - translocated - less than 100 m away, they return to these sites, demonstrating strong homing* capacities in relation to the ability to memorize the places well.

Faithfulness to its habitat may also be relative, as evidenced by pajama cardinalfish, Sphæramia nomatoptera, which, when moved 250 m across coral reefs, do not all return to their original habitat, with 50% of them joining other social groups to which they then remain faithful.

Bibliography: Fish Fish., 2003, 4: 235-246, ICES J.Mar.Sci.Adv.Access,...