One of the longest running controversies in artificial reef research has come to be known as the ‘attraction versus production’ debate. This concerns the high densities of fish often found around artificial reefs (in many cases greater than those around natural reefs). The question, essentially, is this: are these high densities the result simply of attraction of fish from the surrounding areas (which might be undesirable), or do artificial reefs actually provide additional useful resources which translate into increases in fish biomass?
Like many debates in biology, there’s no single definitive answer, and the real situation is likely to be that both mechanisms apply to differing degrees in different systems. Generalising about artificial reefs is notoriously difficult, given the huge variety in types of reef, location, construction materials, design and size – not to mention the differences in the species which might use a reef. Generally, however, it is assumed that attraction is likely to be more important for recruitment-limited*, mobile, pelagic species and increased production may be more important for territorial, reef-inhabiting, habitat-limited* species.
Increased production of fish biomass due to artificial reefs might take one of two forms. It could take the form of population growth, but this is only likely to be relevant for habitat-limited species. The alternative is increased ‘somatic’ production – that is – increased growth of individuals in a population, which might result from the provision of additional food resources by an artificial reef. Fish reproductive success is often associated with size – so it stands to reason that this might also translate into increased future reproductive performance, giving a double bonus to fish populations.
One means of investigating these questions is to consider the diet of fish from artificial and natural habitats as well as investigating the ‘condition’ of these fish. One study published last year did rather a neat job, I think, of this for some artificial reef systems in the waters off California. They compared two offshore oil platforms (artificial reefs) with two natural rocky reef outcrops. The results were interesting in a number of respects.
Firstly, there were differences in the assemblages of small mobile invertebrates. On the natural reefs a greater proportion of potential prey animals were non-amphipod species, whereas amphipods (small shrimp-like crustaceans) were more numerically dominant on platforms. Within the amphipods, there were further differences. The platforms had large abundances of certain exotic (invasive) amphipod species (absent from or very rare on the nearby natural reefs). It’s worth making note of the fact that these structures provide suitable habitat for invasive species, and that this might be a concern (I’ll try to explore this thought in another post at some point).
Why might platforms have different species compositions than reefs? Well, platforms are built from different material than natural reefs and they have a very different structure. Platforms extend from the seabed, vertically up to the sea surface, so a high proportion of the available surfaces are vertical instead of horizontal or more gently sloping (as on most natural reefs). They are also often quite far offshore, and are very young habitats compared with natural rocky outcrops. All of these factors contribute to differences in species composition between reef types.
What effect might these differences have higher up the food chain?
At first glance, not a lot; the diet of painted greenling (Oxylebius pictus) did not vary consistently between the two types of reef site. There was a strong selection for amphipod prey over other food types (meaning that fish were eating more amphipods than you would expect based on their abundance relative to other food types). This might reflect an active effort on the part of fish to seek out amphipods, or they might simply be easier prey. Furthermore, among the amphipods, there was again strong selection, this time for caprellids over gammarids (two families of amphipod species), probably reflecting comparative ease of capture of caprellids. One of the invasive species, mentioned earlier, which did particularly well on the platforms was a caprellid <i>Caprella mutica</i>. Since caprellids are already a preferred prey, we might expect that where an invasive caprellid is particularly abundant, painted greenlings would do well.
And this expectation is borne out. Body condition of greenlings (in this case using a formula which provides an index relating weight to body length; fish with a greater weight at a certain length are in better ‘condition’) was found to correlate with density of prey items, and one of the platforms (which had up to 100x the amphipod density of the other sites) had the best ‘condition’ fish.
What does this, then, demonstrate? I think it provides a good example of artificial reef-based resources contributing to somatic production (via increased fish weight per length) of a fish population. Clearly, the platforms here were at least as good as the natural reefs as food sources for this fish species.
Still, this is one species, in one system, assessed very locally. Other studies have looked at more regional scale effects – to see if the presence of such structures might affect production over wider geographical ranges. As mentioned before, generalising is not recommended for artificial reefs. Certainly not until more studies have found similar results (or not) in other systems.
In any case, this paper is a worthwhile contribution to the literature.
*Habitat limitation is a simple concept – it refers to dependence on a certain critical habitat, such that decreasing the availability of that habitat reduces the population, while providing more habitat allows it to increase. Recruitment-limitation is a little more complicated. Many fish spawn by shedding their eggs into the water column in huge numbers; the resultant larvae are carried by currents until they develop into juvenile fish, suffering huge mortality. Recruitment is the process whereby these juveniles ‘recruit’ into populations of adults. The population size of a recruitment-limited species is therefore limited by the supply of recruits (which fluctuates), and changes in habitat availability are unlikely to affect the size of the resultant population.
Page, H. M., Dugan, J. E., Schroeder, D. M., Nishimoto, M. M., Love, M. S. & Hoesterey, J. C. 2007. Trophic links and condition of a temperate reef fish: comparisons among offshore oil platform and natural reef habitats. Marine Ecology Progress Series, 344, 245-256. doi:10.3354/meps06929