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Studies on polyculture have usually investigated the consequences of this widespread culture method for growth and survival of the cultured species. However, research into the behavioral mechanism underlying competition between co-cultured species is lacking. For the first time in co-cultured fish and crustaceans, this study explored experimentally the behavioral interaction between red tilapia and red-claw crayfish in the context of food competion. The effects of the presence of heterospecifics (absent or present), the size of fish relative to crayfish (larger or smaller) and the number of food patches available for the fish (one- versus two-patch conditions) on the foraging decisions and aggression of fish and crayfish were tested. Time spent by fish in the bottom food patch (accessible for fish and crayfish) was shorter in the presence of crayfish, for small but not large fish and when there was an alternative patch for the fish (accessible only for fish). Time spent by crayfish in the bottom food patch was reduced in the presence of large, but not small, fish and when there was no alternative food patch for the fish. Fish were most aggressive towards conspecifics whereas interspecific aggression was exhibited only by crayfish. At the individual level, the dominant fish and crayfish spent more time in the bottom patch and performed more aggressive actions than the other conspecifics. In fish, this was not altered in the presence of crayfish. In crayfish, the overall reduction in foraging duration and aggression was due to an effect of large fish on the dominant individual. The implication of these findings for fish–crayfish communal culture is discussed.

In the last few decades, continuous attempts have been made to develop methods and technologies to increase fish production. However, unlike other research fields related to aquaculture, acoustic ranching of fish has never attracted intensive activity. Research and developments have been sporadic and lacking in international collaboration. In this review, we describe acoustic sea ranching studies and the progress made in the last 40 years, since the foundations of this method were laid in Norway and Japan. We searched for conclusions drawn by various researchers around the world which might indicate that the concept of acoustic ranching has completely failed, essentially reaching a dead end. Our conclusion is that it has not. On the contrary, there are solid indications that it bears great potential. We discuss the major hurdles which science and technology must overcome in order to achieve significant breakthroughs toward successful applications.

Fish ranching using acoustic conditioning and an automatic fishing machine was tested in a reservoir as an intermediate stage between laboratory studies and future application of sea-ranching technology in the open sea. A self-operated floating fishing machine was used. Fish were conditioned to associate an acoustic signal with food and then were released into the reservoir. Reinforcement of the conditioned response by periodic training was used to maintain the trained behavior. Fish were captured by calling them to the fishing machine using the acoustic signal and automatically closing a net around them. In one experiment, fish responses to the presence of the fishing machine, the acoustic signal and the food were tested. In another experiment, 65% and 13% of stocked common carp ( Cyprinus carpio ) and tilapia ( Oreochromis hybrid) were recaptured, respectively, together with con- and hetero-specifics of the reservoir. The total number of carp captured exceeded the number of stocked carp (154%) in nine capture events over 3 months. The experiments highlight the importance of choosing a competitive species for sea-ranching applications to be successful.

Periodic retraining of St. Peter's fish for retention and reinforcement of an acquired acoustically conditioned behavior after an initial training period of 28 days was studied. Four periodic reinforcement paradigms were tested, each lasting 54 days (paradigm 1 × 3: one training event every 3rd day; paradigm 2 × 6: two training events every 6th day; paradigm 3 × 9: three training events every 9th day). The objective was to study the effects of time interval between reinforcement sessions and number of training events comprising a reinforcement session. No significant differences were found between the three paradigms: all resulted in a similar fish response. Since it had been shown (in our previous work) that acoustically conditioned behavior of St. Peter's fish is totally lost after 6 months, the ability to maintain their responses for this period using a reinforcement paradigm of retraining every 17 days with six training events (three per day for 2 consecutive days) was tested. Fish response was indeed maintained at a high level throughout the 6-month period. The study was conducted in the context of sea ranching in which classical conditioning is used to gain control over fish location and attract them to a feeding location. Such a concept is feasible if the fish can maintain a trained behavior for the long grow-out periods required for growth to marketable size. The present study proves that periodic reinforcement is a feasible method for retaining classical conditioning and maintaining control over the fish for long periods.

Rearing crayfish in individual compartments could be a means of overcoming social-dependent density limitations in order to increase yields per unit area in intensive culture. Two experiments were conducted to evaluate the impact of neighbors on the growth of redclaw crayfish (Cherax quadricarinatus) males in three-dimensional units, consisting of 144 individual compartments, designed for intensive battery culture. The first experiment revealed significantly lower growth rates in crayfish surrounded by neighbors on all sides than in those reared with no neighbors in adjacent compartments, or with neighbors only in compartments on the same level but not above or below them. The second experiment revealed significantly lower growth rates in relatively small crayfish surrounded by large crayfish in a mixed-size unit than in those surrounded by similar-sized small crayfish in a uniform-size unit. No such effect was found for relatively large crayfish surrounded by small crayfish. The results clearly indicated interactions between crayfish in our system, despite them being housed in separate compartments. Growth was adversely affected by the presence of neighbors in adjacent compartments, especially vertically adjacent ones, and the presence of larger individuals in adjacent compartments. This social effect was attributed to partial tactile contact between neighbors in our system. The consequences of these findings for battery culture of crayfish are discussed.

The magnitude of and the sensory involvement in social control of growth in Cherax quadricarinatus were analyzed in the laboratory. In the first experiment 80 pairs of juvenile males, each comprising a relatively small (2.0 g) and a relatively large (3.5 g) crayfish, were raised for 66 days under six different types of social contact. The pairs were raised in full contact with either intact or immobilized claws, separated by a partition with the allowance of transmission of chemical and/or optical cues, and without any contact. Any type of contact with the small one did not affect the growth of the large individual. However, the growth of the small individual was reduced by 50% only when in full contact with the large one, because of both increased time between successive molts and a reduction in the size increment per molt. In the second experiment small juvenile males were reared for 98 days in chemical and/or optical contact with much larger adult males. As in experiment 1, no effect of the chemical and/or optical cues on growth was found. The relevance of these findings to C. quaricarinatus culture is discussed.

Ninety sexually mature Cherax quadricarinatus females were exposed to various combinations of photoperiod and temperature for 2 months during the summer. Females were randomly assigned to either “winter” “semi-winter” or “summer” simulation treatments. In the “winter” treatment, crayfish were exposed to a simulated winter photoperiod (gradual decrease from 14L:10D to 10L:14D, 4 weeks at short day length followed by gradual increase to 14L:10D) and temperature (gradual decrease from 27 to 15 °C, held for 4 weeks, and then gradual increase to 27 °C). In the “semi-winter” treatment, crayfish were exposed to a simulated winter photoperiod and a summer temperature (27–29 °C). In the “summer” treatment, the crayfish were exposed to summer water temperatures and a photoperiod of 14L:10D. Following the 2 months of conditioning, the females were stocked for 7 months in small groups with males under environmental conditions similar to those of the “summer” treatment. All females were individually tagged and molting and spawning events were monitored. Females exposed to “semi-winter” conditioning in the summer, demonstrated a threefold increase in the rate of first spawning during the winter (December–February) compared with the other females. Crayfish breeders can easily implement these findings since shifting the breeding season into the winter only requires shortening of the photoperiod in the summer. The stocking of ponds in the spring with large nursed juveniles that hatched from eggs spawned in the winter, would allow the attainment of market size at the end of the limited growout season in temperate zones.

Interspecific competition for food in polyculture is enhanced in intensive systems because the cultured species mostly depend on artificial feed. This study evaluated the effectiveness of spatially and temporally separating the feed for red hybrid tilapia and redclaw crayfish (Cherax quadricarinatus) as a means of decreasing interspecific competition and increasing the performance of each species in communal culture. The effects of the spatial and temporal separation of feed (i.e., feeding with sinking and floating pellets vs. sinking pellets only, and by day and night vs. day only, respectively) were tested in a 2×2 factorial design. Two 6-week experiments were conducted involving inverse fish–crayfish size relationships. Monocultures of each species were included as controls. No effects of the spatial and temporal separation of feed on the survival and growth of tilapia were found. However, tilapia grew better in duoculture than in monoculture. The performance of redclaw was generally better in monoculture than in duoculture with tilapia. The magnitude of the deleterious effect of tilapia on redclaw was size-dependent, as reflected in reduced survival, growth and foraging of small redclaw in the presence of relatively larger tilapia. There was a significant effect of the temporal separation of feed on the growth of small crayfish; feeding them at night increased their growth by 32%. The temporal separation of feed, in which redclaw were fed at night and tilapia during the day, proved to be an appropriate feeding strategy in their communal culture.

Methyl farnesoate (MF), the predominant juvenile hormone-like compound of crustaceans, was found in the hemolymph of female Cherax quadricarinatus crayfish. Administration of MF to C. quadricarinatus females during their winter reproductive arrest period had no effect on reproduction; however, it did have a tendency to accelerate molting. However, since MF caused increased mortality (∼47% survival in the high MF treatment), we were not able to draw definitive conclusions regarding its physiological affect. In contrast, administration of pyriproxyfen, a juvenile hormone analog, did not cause significant mortality (95% survival in the high pyriproxyfen treatment), although it accumulated in high quantities in the hepatopancreas and, to a lesser extent, muscle tissue, ovaries and gills. The highest dose of pyriproxyfen used in this study, 20 μg/gram animal body weight/week, caused a delay in spawning, which became statistically significant from the seventeenth week. This dose of pyriproxyfen caused a tendency of acceleration of molting without any effect on molt increment. The results of this study show that pyriproxyfen does not seem to be toxic to the crayfish, even in relatively high doses, and might affect the energetic balance between molt and reproduction.

Previous studies on reproduction in the red claw crayfish, Cherax quadricarinatus , have successfully employed broodstock densities of up to 20 crayfish m −2 . This study attempted to increase the density of broodstock in order to intensify production of young in indoor hatcheries. Adult crayfish were stocked in 0.3 m 2 tanks at a 1:5 M:F sex ratio at three densities: 6, 12 and 18 crayfish per tank (corresponding to 20, 40 and 60 m −2 ). The number of berried females was noted at weekly intervals during 14 weeks. There was no difference among densities in survival rate and percentage of spawning and molting, nor in clutch size (pleopodal egg counts) of females. The results indicated that breeding red claw females are insensitive to crowding over a wide range of densities. They suggest that a broodstock density higher than 60 crayfish m −2 is feasible under indoor hatchery conditions.

The annual spawning and molting cycles of the Australian red-claw crayfish was studied in the laboratory over a period of 13 months. Sexually mature crayfish were maintained in 140-1 tanks, each containing four females and one male, under a constant temperature (26–28 °C) and either an ambient or a controlled (14 L:10 D) photoperiod. A similar annual pattern of spawning and molting was evident under the two photoperiod regimes. Females spawned three times and molted twice a year on average. Most spawning occurred during spring and summer, and molting occurred mainly after the breeding season but also between spawns. Variable sequences of spawning and molting were evident during the breeding months in each photoperiod condition. The most common sequences were spawn-molt-spawn and spawn-spawn-molt, with the females as likely to spawn as to molt following the first spawning. An intervening spawning elongated the time interval between molts but did not affect the molt increment. The number of juveniles per spawn was positively correlated with female size, whereas juvenile weight was not. There was an increase in fecundity between successive spawns in females from the controlled, but not ambient, photoperiod, probably due to difference in their spawning history (novice and experienced spawners, respectively). The environmental manipulation applied here failed to maintain continuous spawning over the year in Cherax quadricarinatus broodstock.

The effect of unilateral eyestalk ablation on reproduction, molting and quality of offspring was investigated in Cherax quadricarinatus females during the reproductive-arrest period (winter). At the end of the Autumn, females that had spawned for the first time had relatively uniform, early vitellogenic ovaries, while most females that had spawned previously had ovaries containing two distinctly sized and coloured oocyte populations. These included one population of small oocytes, similar to oocytes found in first-time spawners, while the other comprised of large oocytes. Winter reproduction of females that had spawned previously was not affected by eyestalk ablation, whereas significantly increased spawning activity was observed in eyestalk-ablated first-time spawners. Both females that had spawned previously and first-time spawners that were eyestalk ablated molted significantly more frequently than intact females. The size and number of offspring were not affected by eyestalk ablation. These results show that small synchronous oocyte populations were affected by eyestalk ablation while large oocytes were not affected; females with single-sized-oocyte ovaries tended to be more receptive to the treatment. Unilateral eyestalk ablation may present a solution for induction of spawning only in young females and perhaps in synchronized broodstock.

The effect of ration and spatial distribution of feed on survival, growth, and competition was studied in juvenile red-claw crayfish in the laboratory. Groups of 60 juveniles were raised for 27 days in each combination of ration and spatial distribution: feed being provided once a day or once in 4 days, spatially dispersed or clumped. Aggressive interactions among juveniles were recorded on the 12th day, both prior to and following feed administration. The final weight was much higher when the crayfish were fed once a day compared with once in 4 days, but the respective survival was lower. The latter was explained by increased cannibalism due to higher moulting frequency. The effect of spatial distribution of feed on growth and survival was marginal. However, this factor affected competition for feed. This was revealed by the sharp increase in frequency of aggressive interactions following feed insertion, observed only in the groups where the feed was clumped. Ration had no influence on competition. The weak association between feed competition and both survival and growth is discussed.

Bi-weekly recordings of size and morphotype of individually marked Macrobrachium rosenbergii raised in large communal cages support the “leapfrog” hypothesis, i.e. the orange-clawed male (OC) metamorphoses into the blue-clawed male (BC) only after becoming larger than the largest BC male in its vicinity. This results in a series of differently sized BC males, whose size is positively correlated with the time of their metamorphosis. This growth pattern is achieved mainly through a delay in the transition from the fast growing OC morphotype into the slow growing BC one. This delay may be due to social interaction among males, as males isolated in small cages did not follow this pattern. The function of the “leapfrog” growth pattern is discussed.