The utilization of the Norwegian coastal zone is rapidly increasing and the aquaculture industry faces the challenge of competing for space with other users. To effectively take the advantage of available areas it is thus necessary to realize Norway’s potential for aquaculture. In this context the carrying capacity is crucial.

Develop methods to determine the carrying capacity

Norwegian aquaculture is totally dominated by the production of Atlantic salmon and rainbow trout. The development of a diversified aquaculture industry, which produces several species, has a high priority in the industry as well as with the authorities. This project aims to develop methods to determine the carrying capacity for intensive fish farming and two extensive forms of culture, cultivation of shellfish and sea ranching lobster. The latter form i.e. sea ranching is in its early stage compared to fish farming and shellfish cultivation. The research focus of the different work packages reflects the different level of industry development.

The carrying capacity can be defined as the total number of individuals of a population that a given environment can sustain (Levinton 1982). A more detailed definition with relation to aquaculture is that the carrying capacity of an area refers to the potential maximum production of a species or population that can be maintained within an area in relation to the available food and environmental resources (Rosenthal et al. 1988). The carrying capacity will thus eventually limit the production in an area, either due to perturbations of the environment or to overexploitation of the food source.

WP 1 - Carrying capacity for intensive fish farming

A viable and growing intensive fish farming industry is dependant on sites and recipients with a capacity for dispersion and degradation/consumption of organic waste and nutrients from the farms. The water renewal in the net cages must ensure sufficient water quality, and the farming practice must not result in unacceptable effects of organic waste products and nutrients on the water, sediment or at the fauna in the vicinity of the farm or in the recipient.

Capacity in areas with fish farming

Since the feed for intensive fish farming is collected outside the area where the farming takes place, it is more appropriate to use the concept of holding capacity defined as: “the potential maximum production which is limited by a non-trophic resource” and production capacity defined as: “the maximum tonnage level that can be attained without producing a negative impact on the environment and on the farmed stock“ rather than carrying capacity (Rosenthal et al. 1988). When a fish farm is placed in a fjord or another marine area, the question is whether there is sufficient holding or production capacity left in the system. Norwegian fjords are mostly oligotrophic and so far the widely spread fish farms have not been exceeding the limits of their capacity. However, an expansion in the fish production or a change in the location strategy from widely dispersed farms to areas designated for fish farming may change the picture.

The production capacity in a fjord with regard to organic material will depend on the characteristics of the fjord, such as depth and current, and of the organic material, such as amount and sinking velocity. The sources of organic material in the coastal zone are many such as sewage, phytoplankton and macroalgae assemblages. The latter are washed down to the sediment from shallow areas and may contribute substantially to the organic input to the sediment in Norwegian coastal areas. These other sources of organic material may cause the seabed to be oxygen depleted and any additional material from fish farms resulting in the same effect will exceed the capacity of the area. It is therefore necessary to be able to determine the origin of the organic material and to be able to distinguish between the different sources.

Tracing organic material 

The effects of organic material from fish farms on the site, where the farm is the predominant impact, are fairly well known (Gowen et al. 1987; Holmer and Kristensen, 1992; Hargrave et al., 1993; Karakassis et al. 2000). In Norway a system called MOM (Modelling – Ongrowing fish farms – Monitoring) has been developed at the Institute of Marine Research (IMR), which can determine the production capacity of fish farm sites and keep the impact within acceptable limits (Ervik et al. 1997; Hansen et al. 2000; Stigebrandt et al. 2004). However, in the recipient there may be other sources of organic material either from sedimentation of phytoplankton, aggregation of dead macroalgae or sewage. To determine the production capacity amounts of organic waste from the fish farm must be determined as well as the fate of the waste. The organic material may either accumulate in the sediment and be decomposed by the microorganisms or it may be channelled into the benthic food web. The latter will decrease the risk of over-enrichment of the sediments and increase the productivity of the system including commercial species.

The production capacity in a fjord with regard to inorganic nutrients of nitrogen and phosphorous will depend on the characteristics of the fjord. Most Norwegian fjords are threshold areas, often with very deep basins, and a model has been developed, which predicts the effect of nutrients from fish farms on the fjord capacity (Aure and Stigebrandt, 1991). However, the model cannot be applied in the open fjords of Northern Norway or in open coastal areas. The IMR has also developed a model for simulation of eutrophication from aquaculture (NORWECOM, Skogen et al. 1995). It simulates the circulation in fjord systems and connection between the physical environment, nutrients from fish farms and eutrophication.

To determine the production capacity of a recipient and to ensure it is not exceeded one must be able to trace the organic waste from the fish farms in the area and determine how much is channelled into the different compartments of the marine ecosystem. It is essential to be able to track the organic material, both to identify its origin (fish farm waste or other sources) and its fate. This information will be essential for the development threshold levels for the environmental impact from aquaculture.

Fish farm. Photo: IMR.

WP 2 – Carrying capacity of suspension feeding shellfish

Farming and sea ranching of suspension feeding shellfish is a growing industry in Norway (approx. 800 licenses are given). The production of mussels (Mytilus edulis) has increased from a level of less than 500 metric tons per year during the last two decades to 4-5000 tons in 2004. The development of sea ranching of scallops (Pecten maximus) has been prompted by the new Act on sea ranching, since this has launched a substantial increase in seabed areas for culture. There is also increased interest in flatoyster (Ostrea edulis) farming. The expected expansion of mussel farming has been hindered primarily by harvest closures due to toxic algae, low meat content partially caused by high-density mussel stocks, and post harvest logistic factors, which all interact. However, recent production increase and improvements in the industry has demonstrated that development will proceed and a substantial increase in biomass is expected.

Micro-algae, density, depths and seasonal patterns

Growth of suspension-feeding bivalves is largely controlled by food supply, which is affected by seston concentration and composition as well as seston transport rate (Frechette et al. 1989). Food supply is also linked with phytoplankton dynamics (Rosenberg and Loo 1983, Smaal and Stralen, 1991). Suspension feeders such as mussels have a remarkable capacity to filter the water column and to deplete the water of seston, and in culture they may be food limited at high density (Navarro et al. 1991, Strohmeier et al. 2005). Most studies on bivalve feeding and carrying capacity of bivalve culture have been done in areas with relatively high concentration of micro-algae (Grant et al. 1997, Pitcher and Calder 1998, Dame and Prins 1998, Cranford and Hill 1999, Figueiras et al. 2002 Hawkins et al. 2002). Several of these areas are shallow bays with high tidal amplitude and resuspension of organic material, which increases the seston concentration and seston flux. In comparison, Norwegian fjords along the western coasts are considerably deeper and resuspension of organic material is insignificant at depths of suspended mussel farming.

Low seston environments

The biomass of micro-algae follows a seasonal pattern with a period of algal bloom in late winter/early spring and occasional increased biomass in autumn. On regional or local scale blooms may occur from wind generated up welling of nutrient rich deep water. For extended periods the concentration of chlorophyll a (CHL a) is low (< 1-2 mg m-3, Erga et al. 1989), due to nutrient limitation (Paasche and Erga 1988). Hence, Norwegian fjords and coastal waters are considered as low seston environments compared to sites where most studies carrying capacity have been carried out. In low seston environment the currents that supply seston cannot offset ingestion by the bivalves and the carrying capacity is exceeded (Pilditch et al., 2001; Strohmeier et al., 2005). There is a need to understand more about ecological interactions related to bivalve production in low seston environments, which will be crucial for optimising the exploitation of carrying capacity in bivalve production and development of management strategies in Norway.

The development of a commercial bivalve aquaculture industry in Norway is dependant on sound management that must be based on targeting market demands on quality. This requires scientific knowledge on ecological interactions related to bivalve production, both on carrying capacity estimations and measures for localization of farming. Mussel farming is mainly located in coastal fjord systems, of which there is extensive knowledge. There is a need to integrate our ecological knowledge and modeling practice in fjords with the international expertise on modeling of bivalve eco-physiological and particle dynamics.

Approaches to modelling mussels

Beadman et al. (2002) reviewed the existing approaches to modelling mussels with respect to their possible application to the improvement of shellfish management strategies and carrying capacity estimations. They suggest that future dynamic energy budget models (eg eco-physiology of individual mussels) will need to include population level processes to achieve greater confidence in their application to management strategies. Studies on population levels in a suspended culture situation deals mainly with raft systems.

Based on the study of seston depletion in Norwegian water (Strohmeier et al. 2005), our research group has started modelling on how farm design reduces current speed and influences carrying capacity mussel farms (Aure et al. in prep). This work is done in conjunction with research on benthic community impacts from suspended mussel farming (Strohmeier et al. 2004) and scallop sea ranching (Strand et al. 2005). To achieve high quality shellfish and to select new suitable farming sites (or expand existing farming sites) we need reliable estimates of the carrying capacity, i.e, the stocking density at which production levels are maximized without negatively affecting growth rate (Carver and Mallet 1990).

We expect that the development of models as management tools of suspension feeding shellfish need to include processes at individual/eco-physiological level, farm/population level and ecosystem level (Beadman et al. 2002). In the following these levels are termed Mussel model, Farm model and Fjord model.

Mussel farm in Hardanger. Photo: Ø. Strand, IMR.


WP 3 - Carrying capacity in sea ranching of European lobster

Most benthic marine invertebrates have a life cycle characterized with a pelagic phase followed by settlement onto a preferred substratum or habitat (Linnane et al. 2000 and references therein). The duration of the pelagic phase of European lobster (Homarus gammarus) vary from 2 to 4 weeks depending on temperature. The benthic habitat to which it recruits still remains unknown despite numerous research efforts (Robinson & Tully 2000, Mercer et al. 2001). Studies on H. americanus indicate a strong preference of the early benthic phase to settle in a complex habitat such as cobble, and the preference is related to predator avoidance (Wahle & Steneck 1991, 1992). Average population densities (juveniles) have been estimated to be as high as 6.9 individuals/m2. In European lobster, Robinson & Tully (1999) made an overall estimate of 4.8 individuals/m2 to by a rough approximation of the saturation density i.e. the carrying capacity. However, how this estimate was reached is not clear.

Carrying capacity in the release areas

The new law, Act on sea ranching encompasses the “release and recapture of crustaceans, molluscs and echinoderms”. By April 2004 there were 24 applications for sea ranching lobster and scallop, for 31 localities in total. The applications extended from Tysfjord in north to Risør in south, and varied in area from 0.001 to 21 km2. An unalterable condition set by the Act is related to environmental concern. Increased knowledge on the effect of the sea ranching activities is crucial to fulfil the conditions set by the Act. In this respect, the carrying capacity in the release area is important to be able to understand how densities influence survival and growth of the released animals, and to minimize emigration to surrounding areas. The carrying capacity in sea ranching lobster is also crucial to the economics of the rancher. Licences to ranch lobster in Norway were issued in 2004/2005.

Shelter, lobster density, predation and food availability

Kvitsøy Islands is important due to the knowledge obtained in previous research activities, and for this reasons the area is of particular interest for commercial activities. Bremanger is another interesting area where Grotle Havbeite A/S has received a ranching licence. Carrying capacity is associated with the maximum production of marked-sized lobster, without any irreversible environmental changes. This will depend on a number of factors as shelter, lobster density, predation and food availability (Svåsand et al 2004). With regard to earlier harvest in Norway, the actual densities of lobsters must have been very high. In one region, Tysfjord in Nordland, the present densities of adult lobsters seem to be comparable to the 1960s (Agnalt et al. 2004). Here the population is scattered  along a narrow shelf, and the estimated densities are made as number of lobsters per km beach length, giving about 55 lobsters per km. Estimates for carrying capacities in lobster ranching areas are lacking, but must be expressed as density per square m. There is an urgent need for research in this field.

Survival of released juveniles

At present, however, the limitations in lobster ranching production are connected with early survival of released juveniles. The production within a licensed area is, of course, dependent of the number of released juveniles that actual survive the critical juvenile stages and grow to marked size. Thus the critical carrying capacity of a given ranching area or environment is the maximum juvenile density that can be establish under given environmental conditions. Since the natural substratum is unknown for the European lobster, release experiments with hatchery-reared juveniles have mainly been done into habitats known to sustain adults. Linnane et al. (2000) made some experiments looking at natural substratum preferences of EPB European lobster, by offering the choices of pure cobble, mussel, sand or coralline algae. The release density was 100 stage IV juveniles/m2, and after 1 month the highest settling density of 21/m2 was in mussel/shell. However, the density in mussel decreased through time but remained more or less constant in the cobble substratum at about 18 animals/m2. Jørstad et al. (2001) run a similar experiment, although smaller in scale, providing shell sand with a substantial number of shelter as empty scallop and oyster shells, and stones. The release density varied from 120 to 319 animals/m2 and the final density after 4 months varied from 29 to 130 juveniles/m2. The same experimental design was used to assess juvenile fitness comparing wild and cultured offspring (Jørstad et al. 2005).

Optimal conditions

Although, as mentioned above, some rough estimates of carrying capacity of European lobster have been provided, nothing is known in a sea-ranching situation. Optimal conditions related to release site, release density or quality of the lobster to be ranched is to say the least, scarce. Sea ranching is an extensive form of aquaculture and has through the Act and licences given become a commencing industry. The area occupied by sea ranching is clearly a challenge, along with intensive forms as farming salmon or cod, for the effectual utilization of available areas along the Norwegian coast. The firs step in the progress of combining different forms of culture is to estimate the carrying capacity for each cultivation form. This work package aims to estimate carrying capacity in sea-ranching lobster. Further, in this proposal the close co-operation with the industry will ensure that the results obtained will be implemented directly. This project will produce the first estimates of carrying capacity in lobster ranching in Norway, and the knowledge acquired will be fundamental also to evaluate carrying capacity in other areas.

Lobster. Photo: A.L. Agnalt, IMR.