REPRODUCTIVE DYSFUNCTIONS IN CAPTIVE FISH

Total or partial inhibition of reproduction in captivity All fish species held in captivity exhibit some degree of reproductive dysfunction. It is normally the females who exhibit more serious reproductive problems. These dysfunctions depend on the species and can vary from a total absence of spawning to significant reductions in the quantity and quality of the eggs and sperm produced. What are the reasons? The reproductive problems detected in captive fish are derived from two causes: the stress associated with captivity; the absence of appropriate environmental signals permissive for reproduction. Consequences Total or partial inhibition of reproduction in captivity

Female dysfunctions can be classified in three main types inhibition of vitellogenesis. In this species, reproduction is blocked at very early stages of development. Physiologically, this is the most serious reproductive disorder. 2. inhibition of the process of FOM. Vitellogenesis is completed correctly, but post-vitellogenic oocytes are unable to undergo FOM and become atretic. Atresia can affect the whole population of postvitellogenic oocytes of the gonad, causing the total absence of spawning, or may affect only part of the post-vitellogenic oocytes, which finally causes a reduction in the number of eggs released. The diminished egg production can be slight, or can be dramatic, causing only sporadic spawning of a few eggs. This second type is the most common reproductive dysfunction and is detected in the majority of fish species reared in captivity.

3. inhibition of spawning 3. inhibition of spawning. Spawning is blocked and the ovulated oocytes remain in the ovarian or abdominal cavity. This is, physiologically, the least serious of all reproductive dysfunctions, as only the spawning event is inhibited from the whole reproductive process, although the end consequence is similar, the absence of spontaneous spawning. This dysfunction is observed in salmonids and some flatfishes. In these species, manual removal of the eggs (i.e., stripping) is required. If stripping is not performed, then the eggs degenerate and are reabsorbed, but in such situations they may cause the death of the female.

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Although reproductive disorders are more common and serious in females, male fish also display some important problems. These are usually less important than in females and except in rare cases, males of all fish species usually are spermiating in captivity. The reproductive dysfunctions detected in captive male fish are diminished sperm volume and diminished milt fluidity, which can affect negatively the success of egg fertilization. Diminished sperm production in male broodstock represents a serious problem for those species in which hatchery production is based on artificial fertilization and the acquisition of gametes by manual stripping.

Possible solutions Stress associated with captivity The negative influence of stress should be minimized by appropriate broodstock management (fish manipulation, fish care, prophylaxis, etc.) and adequate culture conditions (tank design, water supply, light intensity, etc). This should be adapted to each species, considering that resistance and adaptiveness to stress varies greatly among species. Absence of appropriate environmental signals The broodstock manager should learn as much as possible about the reproductive biology of the species in its natural habitat and try to adapt the culture conditions to the natural situation. Not working? If reproductive disorders persist even after taking maximum care to reduce the negative effects of these parameters, then the use of hormonal treatments can overcome reproductive problems.