Limnol. Oceanogr., 44(5), 1999, 1352–1357

Light is an important limiting factor for the visual foraging process in fishes, and the light regime may potentially affect the competition between visual and tactile predators. We investigated two equal-sized fjords of quite different pelagic food web structure. Earlier studies have revealed that the jellyfish Periphylla periphylla dominates Lurefjorden, while fish predators dominate in the other fjord, Masfjorden. Furthermore, the mesozooplankton stock of Lurefjorden is larger in both total biomass and size of the individuals. Hence, earlier hypotheses linking the competitive advantage of tactile gelatinous plankton predators to smaller-sized mesozooplankton communities are unable to explain the present phenomenon. To see if the difference in the pelagic biota of the two fjords could be associated with characteristics of the light regime, we measured the light absorbance in the basin water of the two fjords. We found that, due to a slightly stronger influence of coastal water in the basin water of Lurefjorden, the exponential light absorbance coefficient below 100 m is two to three times higher there than in other fjords. This results in a reduction in light flux of several orders of magnitude, effectively reducing the possibility of visual foraging. The tactile mode of predation in jellyfish, however, is not influenced, and we hypothesize that the visibility regime has a decisive role in structuring the pelagic food webs of the two fjords. The question why some pelagic ecosystems support large stocks of fish while others are dominated by jellyfish has received much attention in marine ecology. Mills (1995) points out that as world fisheries begin to experience serious declines, it is relevant to recognize that the carnivorous jellyfishes are ubiquitous and are thus opportunistically positioned to utilize secondary production that is ordinarily consumed by fish. It has been hypothesized that while fish forage most efficiently on large forms of zooplankton (Brooks and Dodson 1965), they may be out-competed by jellyfish if the prey stock is dominated by small size classes (Greve and Parsons 1977), and that fish recruitment can fail, due to prey depletion by large standing stocks of jellyfish (Möller 1980). Here we show that neither of these hypotheses are likely to account for pelagic food web differences observed among fjords. Instead we argue for an alternative hypothesis recently worked out in theory (Eiane et al. 1997): that poor visibility in the water column may prevent the visually foraging planktivorous fishes from obtaining the foraging rates required for population maintenance while tactile planktivores, such as jellyfish, are not affected. Light limitation in visual foraging—The present work and the study of Eiane et al. (1997) emerged as a result of theoretical and experimental work on how predation by fish is affected by light and optical properties of prey and environment (Aksnes and Giske 1993; Giske et al. 1994; Aksnes and Utne 1997; Utne 1997). Furthermore, field studies on the mesopelagic fish Maurolicus muelleri, an important zooplanktivore in Norwegian fjords, have revealed that both the feeding and the vertical behavior are highly sensitive to the prevailing light conditions of the water column (Giske and