Effective Spawning Strategies for Producing Viable Fathead Minnows Embryos for Use in Fish Embryo Toxicity Tests

Abstract

Traditionally, the toxicity of chemicals and industrial effluents is evaluated using a 7-day fathead minnow (Pimephales promelas) larval growth and survival (LGS) test. However, there has been a push to adopt an alternative toxicity testing method, the fish embryo toxicity (FET) test, which utilizes fathead minnow embryos rather than larvae. The adoption of the FET test as a replacement for the LGS test is considered to represent an improvement in animal welfare as embryos are thought to be less sensitive to pain and distress than older organisms. Implementation of the FET testing protocol has encountered some challenges, namely, the difficulties associated with obtaining the number of embryos required to initiate the test. To aid in the adoption of the FET test, this study sought to optimize the breeding output of fathead minnows through manipulation of male: female ratios and breeding structure availability. To identify which sex ratio produced the largest number of embryos, breeding output was monitored in minnow breeding colonies with minnows arranged in 1:2, 1:4, 2:4 or 2:8 male: female ratios with limited (2 hr.) or unlimited (24 hr.) access to breeding structures. Over the course of 28 days, when access to breeding structures was limited, the 1m: 2 ratio produced a mean of 9.9 embryos/colony/day. The 1m: 4f ratio produced a mean of 22 embryos/colony/day. The 2m: 4f ratio produced a mean of 1.8 embryos/colony/day. And, the 2m: 4f ratio produced 3.7 embryos/colony/day. This trend was similar when access to breeding structures was unlimited with embryo counts respectively at 33.9, 58.3, 35.4, and 42.2 embryos/colony/day. The results of this study demonstrated that spawning was maximized through use of a 1:4 male: female ratio. Additionally, it was determined that more uniform embryo development was observed with limited access to breeding structures. By using a 1 male: 4 female sex ratio and providing limited access to breeding structures, investigators will be able to optimize fathead minnow embryo production, thus making the process of FET initiation more efficient.