Effects of Light on Schooling and Feeding of Jack Mackerel, Trachurus symmetricus

102

Engraulis capensis (Gilchrist) is capable of both filter and particulate feeding, the latter being the dominant mode. Food concentrations required to initiate filter feeding were inversely related to particle size. Turning rate, swimming speed and feeding mode also depended on prey size. The anchovy switched from filtering to biting at a threshold prey size of 0.710 to 0.720 mm; the feeding mode employed by the fish on prey approximating this size depended upon prey concentration. Biting clearance rates were greater than those for filtering, and the fish fed at maximum rate over most of their prey size spectrum. These results indicate that E. capensis had adapted its feeding behaviour to maximise food intake and to minimise energy expenditure. The selective grazing of E. capensis may have a significant effect on the structure of its prey communities.

Section: Discussionmentioning

confidence: 96%

Effect of particle size and concentration on feeding behaviour, selectivity and rates of food ingestion by the Cape anchovy Engraulis capensis

James¹,

Findlay²

1989

102

“…Light intensity varied considerably throughout the survey and also exhibited a significant positive effect on larval condition factor, which was calculated from independent samples. Many fish larvae, incl.uding herring (Blaxter & Batty 1990), cannot feed in the dark and consequently feeding is liinited by light intensity, as demonstrated in several laboratory (Hunter 1968) and field (Gilbert et al 1992) studies. Light intensity may have a positive effect on growth through a variety of mechanisms.…”

Section: Discussionmentioning

confidence: 99%

Environmentally induced short-term variability in the growth rates of larval herring

Gallego¹,

Heath²,

McKenzie³

et al. 1996

ABSTRACT. The growth of hernng Clupea harengus larvae w~thin a patch tracked for approximately 20 din the Orkney-Shetland area (north of Scotland) was ~nvest~gated using otollth microstructure and, in a supporting role, condition factor analyses. Daily otolith growth responded conservatively to external factors, there being significant autocorrelation within individual otolith increment sequences. Past growth history explained most of the variation of the data. Both otolith microstructure and condition factor analyses yielded consistent results, indicating that short term variations in growth rate were related to environmental factors such as wind-induced turbulence level, and changes in illumination and prey concentration A dome-shaped effect of wind-generated turbulence on larval growth was detected, which indicated a maximum response at a wind speed of approximately 14.5 m S-', a value very close to the speeds predicted by theoretical studies to generate maximum ingestion rates of larval fish.KEY \VORDS: Fish larvae. Growth Environment. Turbulence Otolith INTRODUCTIONThe survival of fish larvae under natural circumstances is largely dependent on stage-specific mortality rate, i.e. the interaction between growth and mortality (Houde 1994). These 2 processes are interrelated since survival over any development interval is the product of mortality rate (d-l) and stage duration (d). Mortality rate generally decreases with increasing body size due to a variety of size-related effects on predator-prey interactions (Fuiman 1994), and hence the cumulative mortality for a population of fast growing individuals is less than that for slow growing individuals subject to the same predation rate (Houde & Schekter 1980, Houde 1987.Early attempts to detect growth varlability in fish larvae and relate it to environmental features were, with a few exceptions, unsuccessful (Heath 1992). Investigators examined the mean growth of the population, usually averaged over time periods of days or weeks, in relation to large-scale spatial (10 to 100 km) differences in prey abundance or environmental characteristics. However, this large-scale variation in time a n d space may account for only a small part of the total variability in growth. Small-scale spatlal and temporal variability, which could be more important for larval survival, is not examined in this approach. For example, small-scale turbulence may vary on time scales of minutes to hours and over short distances, particularly in the vertical plane. It is now evident that small-scale turbulence may have a profound influence on the encounter rate and capture success of planktonic predators (MacKenzie e t al. 1994). Thus there is scope for considerable within-population variability in growth induced by ingestion rate variability, over and above additional sources of variation between individuals due to foraging experience, genetic differences, etc. Studies of the effect of individual variability in growth on larval survival have shown that this factor may be as, or more, important...

“…Such behaviour reveals the jack mackerel paradox: this fish has to reach its highest level of school organisation under the lowest light intensity level. However, (1) jack mackerel have high low-light vision capabilities (Hunter 1968), and (2) we still have much to learn about the non-visual stimuli that drive schooling behaviour.…”

Section: Prey Accessibility 'Rings the Bell' For Foraging Timementioning

confidence: 99%

Determinism and plasticity of fish schooling behaviour as exemplified by the South Pacific jack mackerel Trachurus murphyi

Bertrand¹,

Barbieri²,

Gerlotto³

et al. 2006

Pelagic fish schools are thought to show a general pattern of dispersion at night and aggregation within schools during the day. This pattern is often accepted as the major rule driving most of the other physiological, biological and ecological processes. Foraging on mobile prey, for instance, is assumed to be enhanced by schooling behaviour. Current theory assumes then that foraging is only possible for obligatory gregarious predatory fish from dawn to dusk. However, offshore mesopelagic communities perform vertical migrations and are out of reach for most oceanic pelagic predators during the day (with the exception of some apex predators, e.g. swordfish or bigeye tuna). To investigate how fish may overcome this apparent contradiction, we studied the 3-dimensional spatial strategy of the South Pacific jack mackerel Trachurus murphyi according to the abiotic and biotic conditions of the habitat. Data came from acoustic surveys performed in central Chile in 1997Chile in , 1998Chile in and 1999. Our results show that the jack mackerel distribution was driven by prey during the night when foraging, and related to the hydrology when resting during the day in the upper part of the oxycline. Fish were more aggregated at night than during the day, probably because jack mackerel cycles of schooling behaviour depend primarily on prey availability. This 'atypical' behaviour could be an adaptation of gregarious pelagic fish to an oceanic ecosystem. Fish schooling behaviour is not necessarily driven directly by the diel cycle; rather, it can be functional and depends on prey availability.