Failed Induced Spawning in a Catla Hatchery
A carp hatchery in Kamrup holds 8 female catla of 3.5–4.5 kg and 12 males. The broodstock were reared on rice bran and oil cake alone, with no vitamin or mineral mix, and were transported 60 km in open drums the previous afternoon. At 20:00 all females received a single intramuscular dose of 5 mg/kg of pituitary gland extract; males received 2 mg/kg. Water temperature was 27 °C at injection but fell to 22 °C by 03:00 because the overhead tank was left uncovered. At 07:00 none of the females had spawned. When lifted, two females leak a few eggs and show slightly swollen vents. Several males release thin, watery milt. The hatchery manager proposes to re-inject all eight females at 10 mg/kg at 08:00 and to strip whatever eggs appear by 10:00.
1. Should the manager proceed with the re-injection? What examination would you carry out first and what decision rule would you apply?
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Do not re-inject all eight blind. The first step is a physical examination of each female: feel the abdomen for softening and distension, inspect the vent for swelling, redness or egg extrusion, and apply gentle pressure from just behind the pectoral fins towards the vent over a dry bowl. Record the result for each fish individually.
Decision rule. (a) If eggs flow freely and are round, translucent and separate, the fish is ovulated - strip it at once, fertilise with milt from two or three males, and do not inject it. (b) If the abdomen is soft but no eggs flow, the fish is close to ovulation - hold it in flowing water and re-check every 30–45 minutes. (c) If the abdomen is still firm and no eggs appear, ovulation has not occurred, and a second injection may be justified, but only after the expected latency for the actual temperature has elapsed.
Latency reasoning. Injection was at 20:00 and the check at 07:00, only 11 hours. Latency in catla at 27 °C is roughly 5–7 hours, but the overnight fall to 22 °C lengthens it substantially, since the process is driven by accumulated thermal units rather than clock time. At 22 °C, 9–12 hours is plausible. Re-injecting an already-ovulating female at double dose causes over-ripening, egg retention in the body cavity and heavy post-spawning mortality. The manager's 10:00 stripping deadline is also unsafe: eggs stripped before they are fully ovulated will not fertilise well.
2. Diagnose the likely reasons the first round failed, separating management errors from broodstock factors.
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Management errors. (1) A single-dose protocol was used. Catla normally needs a priming dose followed by a resolving dose several hours later, because a single moderate dose may not provide an adequate ovulatory surge. (2) The overnight temperature drop was not controlled; an uncovered overhead tank in a cool night defeats any latency calculation and can arrest the process. (3) The fish were transported in open drums the previous afternoon and injected the same evening, so they were still stressed and possibly not feeding. (4) The manager's instinct to double the dose and strip on a fixed timetable shows a protocol driven by clock time, not by the fish's state.
Broodstock factors. (5) Nutrition: rice bran and oil cake alone provide energy but marginal protein quality, vitamins and minerals; broodstock need a formulated broodstock ration of about 28–30% protein with fish meal, oil cake, rice polish, vitamin and mineral mixture for at least three to four weeks before spawning. Poor nutrition shows up as thin milt in males and poor egg quality in females. (6) The males themselves are suspect: watery, thin milt means a low sperm density, which would reduce fertilisation even if eggs had been obtained.
Conclusion to state. The failure is not one cause but a stack of them, dominated by an inadequate hormonal protocol acting on broodstock in poor condition under an uncontrolled temperature regime. Selecting the single cause is less useful than identifying the protocol and conditioning failures.
3. Set out the corrected protocol for the next attempt, covering broodstock conditioning, dose strategy and spawning management.
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Conditioning (3–4 weeks before spawning). Move the selected brooders to a well-aerated earthen pond or circular tank with gentle flow and keep them separate by sex. Feed a broodstock ration of about 28–30% crude protein - oil cake, rice polish, fish meal and a vitamin-mineral premix - at 2–3% of body weight daily. Do not transport and inject on the same day; allow at least 48 hours of quiet acclimation.
Selection and dose. Choose females with a soft, distended abdomen and a swollen, slightly pink vent; choose males that release thick milt on gentle pressure. Use a two-dose regimen for females, for example a priming dose of 2–3 mg/kg pituitary extract followed after about 6 hours by a resolving dose of 5–6 mg/kg. Give males a single dose of 2–3 mg/kg at the time of the second female injection. Weigh each fish to the nearest 50 g, because dose errors here are a common cause of failure.
Temperature and latency. Hold water at 26–28 °C, cover overhead tanks, and calculate latency in thermal units. Begin checking females about one hour before the expected ovulation time, then every 30–45 minutes. Hold ready fish in flowing water; never strip before eggs flow freely.
Spawning and incubation. Strip eggs into a dry, clean bowl, add milt from at least two or three males, mix briefly with a feather, activate with clean water for a minute, and transfer to hatchery jars within a few minutes. Record dose, weight, temperature, injection time, ovulation time and fertilisation rate; these records are how the next cycle is tuned.