1. Background
The polyculture of several fish species that feed on different natural resources is an important management technique, particularly in extensive big water reservoirs
and semi-intensive ponds of a few acres freshwater culture systems, where it promotes the efficient utilization of the production potential of the pond Lin, 1982;
Chang, 1987; Milstein, 1992. Synergistic interactions among fish species are based on two interrelated processes, namely, the increase of available food resources and
the improvement of environmental conditions Milstein, 1992. The advantages of fish polyculture are manifested in higher growth rates and yields in polyculture than
in monoculture Yashouv, 1971; Hepher et al., 1989. However, fish polyculture also has drawbacks, related to the fact that all fish must be treated in the same
manner, without any special management, specific to each species e.g., differential harvesting according to size, tolerance of low temperatures or to match market
demand.
For optimal marketing it is necessary to sort harvested fish according to species and size. A common practice called ‘pond decimation’ or pond thinning is to
harvest, at specified intervals during the season, fish which are ready for marketing, usually the larger fish of one of the species. Pond decimation increases overall fish
production by reducing the population density, and reduces growth disturbance caused by competitive interaction between big and small fish. It is done by
restricting the fish to a small part of the pond with fishing nets and lifting them onto a sorting table by means of vacuum pumps. A number of workers stand beside
the sorting table and manually collect the fish which are ready for marketing, while the other fish slide back into the pond from a height of a few meters. This is a
labor-intensive and expensive operation and it is harmful to the fish. At the end of the growing season, the pond is harvested and the fish are manually sorted by
species and sized prior to marketing.
To date, growers do not have means for selective fish harvesting which would enable them to harvest fish of specific species and size according to market demand.
The global goal of the present study is to try to develop means and methods for continuous, automatic, underwater selective harvesting of live fish.
Zion et al. 1999 reported results of an image processing algorithm, based on the application of the method of moment-invariants Hu, 1962 to images of common
carp Cyprinus carpio, St. Peter’s fish Oreochromis sp. and grey mullet Mugil cephalus, commonly grown together, to discriminate among the three species and
to estimate fish size. Fish species identification accuracy was 99, 93 and 93, respectively, for grey mullet, St. Peter’s fish and carp average results for three sets
of images. Fish mass was closely estimated from their image area. The correlation coefficients between fish mass and image area of grey mullet, St. Peter’s fish and
carp were 0.954, 0.986, 0.986, respectively. The fish were dead when their images were acquired by a CCD camera in a lighting chamber, under controlled lighting
conditions. They were lain on their side, at various positions and angles, on a black background plate and lit by diffused fluorescent light from the direction of the
camera. One of the conclusions drawn from that work was that since species
identification was based on shape analysis and since surface texture and color were irrelevant, background lighting could be used to improve the contrast between the
fish image and background and thus improve image segmentation. Following that work it was decided to test the ability of the method to identify
the three species while the fish were alive. Rather than inserting a camera into the water in a waterproof casing, fish were placed in an aquarium and their images
were taken by a CCD camera mounted outside. In a possible practical application fish will have to be ‘delivered’ into the field of
view of the inspection system a video camera and lighting system in the present case, preferably one at a time and side-on to the camera. Fish have been effectively
trained to approach feeders from long distances in both marine Fujiya et al., 1980 and freshwater environments Abbott, 1972; Levin and Levin, 1994 in response to
acoustic signals.
Sorting of large fish populations on the basis of fish performance depends upon the large-scale transfer of information from trained to naive fish, since only a small
fraction of the population can be effectively trained. Rapid transfer of information from trained to naive fish, based on imitation and social facilitation in mixed
schools of trained and naive fish has been demonstrated for various training tasks Hale, 1956; Fujiya et al., 1980; Suboski and Templeton, 1989. In the case of
Pagrus major, for example, which were trained in the laboratory to approach feeders and released in the ocean for sea ranching, trained fish were frequently
accompanied by naive specimens when captured Fujiya et al., 1980.
In addition to the development and testing of the computer vision part, a preliminary experiment was conducted to test the ability of fish to be trained to
swim through a narrow Plexiglas channel. Such a channel could be a part of a sorting system, through which fish images could possibly be acquired.
2. Materials and methods
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