Supplement to Vol. 7, No.1: August-September 2013
[Reader: This is the complete interview with John Caddy for MEAM 7:1. For context, it includes the same opening paragraphs as the abridged article that appeared in the newsletter.]
It makes intuitive sense that if a given area of ocean offers several types of seafloor habitat, and if commercially targeted fish populations prefer one habitat over the others (particularly at different life stages), then stock assessments should account for the spatial distribution of the habitats.
However, says fisheries scientist John Caddy, this is too seldom the case. Caddy, a former chief of FAO’s Marine Resources Service, says stock assessments typically ignore habitats — particularly habitats that are uncommon, like those of high structural complexity in shelf waters. Instead, the assessments assume that the habitat across the fishery is homogenous and non-complex. As a result, fisheries are allowed to work in (and potentially damage) areas that are highly-structured, and which may be disproportionately important to the health of a targeted stock.
In a paper published this April 2013 in the ICES Journal of Marine Science, Caddy suggested that fisheries assessments that do not account for uneven spatial distribution of structurally complex habitats are doomed to error (“Why do assessments of demersal stocks largely ignore habitat?”, available for free at http://bit.ly/CaddyAssessments. MEAM spoke with Caddy about his conclusions, and the full interview appears below. Journal references are provided at the end.
MEAM: Our contributing editor Tundi Agardy has called your conclusion the “grand unifying principle of ecosystem-based management”: that is, management must be built on a foundation of ecosystem understanding, which in turn is made possible by the use of physical cues to help identify priority areas. What are your thoughts on that?
John Caddy: The fact is that applying ecosystem-based management requires us to incorporate the complexity of physical structure, as well as the biological components of the habitat into our models. The majority of fishery models now in use do not do this, and hence, strictly speaking, are not ecological models. They tend to assume either that habitats are uniform in fish-producing capability (the “dynamic pool” assumption), or that calorific transfers in food webs alone are a realistic model of what goes on in a marine ecosystem. (How likely is a food shortage to occur for a depleted species?).
Obviously, the quantity of food available is important. But for juveniles of demersal fishes to harvest food organisms safely, suitable cover should be available nearby to protect them between feeding forays. Putting my ideas in their global context, terrestrial ecology has found that the disappearance of complex structures (forests) is the main reason for declines in biodiversity. Not having monitored how our activities affect those structural elements in the sea that are required for life-history completion, we are a long way from a similar perception.
As indicated in Walters and Juanes (1993), the absence of adjacent cover for demersal juveniles next to their food resources drastically restricts the proportion of a food easily available, given that distant excursions across open bottom are unwise in the presence of predators. One could reasonably postulate that a linear measure of the interface between structurally-complex habitats and open bottom (which is very sensitive to the impact of dredges and trawls) should be a good indicator of the potential survival of the juvenile stages of many species! More realistically, incorporating habitat and spatial components into fisheries models seems the way to go, at least for the benthic/demersal resources I was writing about. In addition, in many cases the micro-habitats are fractal in configuration, which has important implications for size selectivity of juveniles. It can be demonstrated that an increase in organism size in fractal habitats drastically reduces suitable cover for them, and leads to their dispersion or migration elsewhere: a high-risk process.
A more general comment is that ecological considerations dictate that for spatial management to be realistic, we must map habitat configurations more carefully. We need to include geological factors (outcrops, sediments, and structural complexity), and this requires underwater mapping capability. The critical habitats encountered must then be protected by introducing spatially focused management measures. A number of papers in the literature have emphasized the importance of restrictions on bottom-towed gear, including the protection of spawning, nursery, and migration routes from incidental damage (see Caddy and Seijo 2011). Experience in the Mediterranean suggests that establishing closed areas (reproductive refugia) for the larger spawners offshore could be an effective management measure. Now that satellite monitoring of fishing fleets is a reality, combining area/resource allocations to fleets in open areas, with realistic penalties for fishing closed areas, could become the norm.
Sometimes taking a look at a phenomenon in the opposite way to the conventional approach can be helpful. Stock-Recruit relationships are widely used in fisheries assessments, but tell us little about which ecological factors are critical to pre-recruits and early recruits. In contrast, I have been interested in knowing more about the more complex interactions within Recruit->Stock relationships which are studied by field biologists rather than by stock assessment experts. Field studies often reveal that the numerical survival of new recruits is in part a function of the spatial extent of nursery areas. Preferred juvenile habitats have characteristics that support successive functional stages of most organisms pass through in ontogeny. Several interesting features emerge from this perspective:
- Many larger demersal fish or motile invertebrates pass through several stages in development after recruiting to the bottom, and each stage may have particular habitat requirements, especially relevant to predator avoidance. Should one of these optimum habitat configurations be in short supply, a “habitat bottleneck” may intervene to reduce the cohort’s survival to the next functional stage. If so, simply assuring a high number of effective spawners does not guarantee a good recruitment.
- Structurally-complex features of the marine habitat are relatively much rarer in the sea than on land. During the pre-recruitment or recruitment stages, these structural forms are available “cover” for predator avoidance, but are rarely considered in stock assessment. It seems to me that if nursery grounds have been regularly dragged by bottom gear, the abundance and density of epifauna/flora, and thus the ability of the ground to support juvenile stages safely, must be greatly reduced. This is not to mention the significant proportion of small organisms damaged in so-called “selective trawling processes” over nursery areas (see Caddy and Seijo 2011).
- I support checking for potential “population bottlenecks” at each life history stage, either caused naturally (due to inadequate areas of critical habitat), or due to dragging. If these shortages apply, they may effectively reduce survival to the next life history stage. An example of a bottleneck on Pedro Bank, Jamaica, emerged from my re-analysis (Caddy 2011) of the size frequencies of reef fishes in John Munro’s unique study on an unfished reef population (Munro 1983). The decline in numbers with size was often consistent with fractal expectation, until a shortage of large crevices in the reefs exposed larger fish to shark predation when numbers caught at size dropped sharply.
- Considering the natural mortality rate experienced in different stages of the life history seems important, and my ICES paper summarizes some ideas on how we may model stage- or size-specific mortality rates, even without bottlenecks. Let us accept the reality that the “constant M” assumption does not apply throughout the life history!
MEAM: Fishers, divers, and other ocean users have long recognized that, quite often, where there is structure there are more fish. Why do you think it has taken fisheries managers so long to build this into their models?
Caddy: On this point, studies of marine ecology incorporating direct observation techniques versus work on fish stock assessment, seem to have little in common. It has long been recognized by fishers and divers that fish/invertebrate populations are contagious in distribution – in part due to the higher fish densities associated with outcrops. This is also relevant to how habitat structure is damaged by harvesting methods such as bottom trawls and dredges sweeping the sea floor. Ironically, technological advances that allow the viewing of outcrops, ship wrecks, oil rigs etc, as well as fish schools, permits trawling closer to obstructions, which profits from the fish-attracting ability of outcrops, but also increases the risk lost gears and “ghost fishing’.
The geometric configuration and structural complexity of habitats must be important, and this points the way to engineered habitat manipulation as an enhancement strategy (Caddy 2007). A reasonable conclusion from extensive work carried out in measuring fractal coefficients of marine outcrops and epifauna/flora, is that most of these have fractal characteristics, even if the fractal coefficient varies spatially. What can be deduced from this is that the number of crevices, spaces and holes drops off dramatically with size, progressively exposing a growing organism to predation. This must be one of the factors contributing to migrations in the life histories of motile marine organisms – a search for habitats with larger crevices suitable for larger organisms! The fact that one can deduce a reasonable vector of natural mortality rate at size from the fractal configuration, points to habitat structure as a predominant factor determining size frequencies of many bottom-dwelling organisms.
MEAM: Your paper points out the fisheries management value of installing artificial reefs in reserves: i.e., to provide more cover and thus supply recruits to adjacent fished areas. Taking this idea to its logical conclusion, would you propose designating very large areas of the shelf as reserves, installing extensive artificial reefs in them, and using these areas as (fenceless) quasi-hatcheries to stock nearby fished areas?
Caddy: I was struck by how Jeffrey Polovina argued against the use of artificial reefs in open fishing areas – by noting that the large holes in such reefs attract the rarer mature fish, making them vulnerable to overexploitation (Polovina 1989). This argument against using artificial reefs to attract larger reef fishes that were already having difficulties finding larger natural crevices seems to me invalid in one case, however. Using artificial reefs with large crevices to convert a small area closed to fishing into a reproductive refugium seems a good idea!
In a broader sense, a concern with cover resources implies that we need to take steps to protect specific habitat types, and this practical application of adding complex habitat structures to the natural environment appears worth studying further.
For more information:
John Caddy, Latina, Italy. Email: jfcaddy [at] yahoo.co.uk
Caddy also publishes a fisheries blog — titled “My Published Work on Fisheries Science” — at www.myfisherywork.com.
Caddy J.F. and J.C. Seijo.(2011). Destructive fishing practices by bottom gears: a broad review of current research and practice. (in English). Ciencia Pesquera (2011) Número especial 19: 5-58.
Caddy, J.F.(2011). How artificial reefs could reduce the impacts of bottlenecks in reef fish productivity within natural fractal habitats. Chapter 4, pp 45-64, In: Bortone, S.A., F.P. Brandini, G. Fabi, and S. Otake. Artificial reefs in fisheries management. CRC Press, Boca Raton, London, New York.
Caddy, J.F. (2007). Marine habitats and cover: their importance for productive coastal fishery resources. Oceanographic Methodology Series, UNESCO Publishing, 253p.
Munro, J.L. (1983). Caribbean coral reef fishery resources ICLARM Stud. Rev. No 7.
Polovina, J. (1989). Artificial reefs: nothing more than benthic fish aggregators. CalCOFI Reports Vol. 30.
Walters, C.J. and F. Juanes. (1993). Recruitment limitation as a consequence of natural selection for use of restricted feeding habitats and predation risk taking by juvenile fishes. Can. J. Fish. Aquat. Sci., Vol 50, pp 2058-2070.