54
7 Final Remarks
7.1 Landmarks and Beacons: Orienting and Meeting
The pelagic environment is characterized by its extensiveness, homogeneity, lack
of refuge and seasonality and patchiness of food availability. Migration is a common strategy for coping with seasonality in the production cycle. The question of
how migrating animals can find their way while covering immense distances, have
long fascinated and puzzled mankind. Migrating in the ocean poses formidable
challenges to marine animals because the open ocean appears as a most uniform
environment, lacking visual cues that are known to be fundamental in guiding
migrations on land. Olson (2002) proposed that fronts may provide clues for largescale migrations. Fronts are narrow regions which represent anomalies in the flow
field and display high contrast in physical as well as biological variables, so their
detection and tracking could be feasible for some migratory species, assisting
migrants to follow a course during their journeys. Because of the high prey abundance at fronts, individuals can feed while travelling making it difficult to disentangle foraging and migration processes.
Group formation is a widespread phenomenon throughout the animal kingdom.
Different reasons for grouping include decreasing the predation risk, promoting
optimal foraging, increasing reproductive success and facilitating migration and
learning. The homogeneous and expansive nature of the marine pelagic domain
favors dispersion. Hence, for gregarious fish species some processes must enhance
the gathering of individuals (Soria et al. 2009). Several fishes (notably tuna) living
in a rather uniform universe are attracted by any physical “anomaly” in their environment (e.g. floating objects, bottom discontinuity, high gradients). The “meeting point hypothesis” Freón and Dagorn (2000) considers that schooling pelagic
species can make use of floating objects to increase the encounter rate between
isolated individuals or small schools and other schools. For an isolated fish, or a
small group of fish, the floating object might be easier to detect than a large school
(using one or several senses: vision, audition, olfaction). In addition, from the
point of view of the energy balance, waiting for conspecifics at a meeting point
is less costly than swimming to find them. The ability of fronts to concentrate
residue floating at the sea surface (e.g. kelp, branches or trunks of tree, vegetal
debris), and any industrial residue (e.g. boards, housing, parts of destroyed fishing
gears) could make them useful meeting points for pelagic fishes.
7.2 Mechanical Energy for Retention
The ocean is a highly dispersive environment. Dispersion is a combination of
advection and eddy diffusion. Advection is the mean transport of a collection of
particles, while diffusion reflects the differences in transport of individual particles. In words of Largier (2003), imagine “a cloud of dye in the water: it moves
7 Final Remarks
7.1 Landmarks and Beacons: Orienting and Meeting
The pelagic environment is characterized by its extensiveness, homogeneity, lack
of refuge and seasonality and patchiness of food availability. Migration is a common strategy for coping with seasonality in the production cycle. The question of
how migrating animals can find their way while covering immense distances, have
long fascinated and puzzled mankind. Migrating in the ocean poses formidable
challenges to marine animals because the open ocean appears as a most uniform
environment, lacking visual cues that are known to be fundamental in guiding
migrations on land. Olson (2002) proposed that fronts may provide clues for largescale migrations. Fronts are narrow regions which represent anomalies in the flow
field and display high contrast in physical as well as biological variables, so their
detection and tracking could be feasible for some migratory species, assisting
migrants to follow a course during their journeys. Because of the high prey abundance at fronts, individuals can feed while travelling making it difficult to disentangle foraging and migration processes.
Group formation is a widespread phenomenon throughout the animal kingdom.
Different reasons for grouping include decreasing the predation risk, promoting
optimal foraging, increasing reproductive success and facilitating migration and
learning. The homogeneous and expansive nature of the marine pelagic domain
favors dispersion. Hence, for gregarious fish species some processes must enhance
the gathering of individuals (Soria et al. 2009). Several fishes (notably tuna) living
in a rather uniform universe are attracted by any physical “anomaly” in their environment (e.g. floating objects, bottom discontinuity, high gradients). The “meeting point hypothesis” Freón and Dagorn (2000) considers that schooling pelagic
species can make use of floating objects to increase the encounter rate between
isolated individuals or small schools and other schools. For an isolated fish, or a
small group of fish, the floating object might be easier to detect than a large school
(using one or several senses: vision, audition, olfaction). In addition, from the
point of view of the energy balance, waiting for conspecifics at a meeting point
is less costly than swimming to find them. The ability of fronts to concentrate
residue floating at the sea surface (e.g. kelp, branches or trunks of tree, vegetal
debris), and any industrial residue (e.g. boards, housing, parts of destroyed fishing
gears) could make them useful meeting points for pelagic fishes.
7.2 Mechanical Energy for Retention
The ocean is a highly dispersive environment. Dispersion is a combination of
advection and eddy diffusion. Advection is the mean transport of a collection of
particles, while diffusion reflects the differences in transport of individual particles. In words of Largier (2003), imagine “a cloud of dye in the water: it moves