Selective Breeding for Hobbyists: Goals, Selection and Welfare

How hobbyists can plan selective breeding of aquarium fish: setting goals, choosing selection criteria, handling non-selected fish, planning generations and avoiding harmful traits.

What selective breeding is

Selective breeding, or artificial selection, means choosing which males and females reproduce in order to develop particular traits. Breeders use inbreeding, linebreeding and outcrossing to reach their goals. Fish respond strongly: in Atlantic salmon, selection has raised body weight by about 30% per generation. Selecting for a single trait can have side effects, however, and reduced genetic diversity makes a stock more vulnerable to disease. Spawning behaviour of individual species is covered in the per-species breeding guides and in the fish reproductive strategies guide.

Setting goals

Traits fall into two kinds. Qualitative traits are classified rather than measured, such as a colour type or normal versus deformed, and are controlled by one or two genes. Most of those described in fish are controlled by a single autosomal gene with two alleles. Quantitative traits are measured, such as length, weight and fecundity. Heritability (h²) is the share of the variation in such a trait that comes from additive genetic variance, the part that selection can exploit. Traits with high heritability can be improved efficiently by selecting individuals on their own merit. Traits with low heritability, which is often the case for viability and fecundity, respond better to selection between families (FAO Fisheries Technical Paper 392).

A narrow, measurable goal is easiest to track. In feral guppies (Poecilia reticulata), three generations of selection on male orange area made high and low lines differ significantly; realised heritability was 1.47 in the high line. Female preference for orange, selected at the same time, responded less clearly (Sato & Kawata 2020). Not every trait moves at the same speed under selection.

Selection criteria

• Cut-off value: set a minimum acceptable value for the trait. Fish that meet or exceed it are kept as breeders, and the rest are culled, meaning removed from the breeding population.
• Individual or family selection: fish can be chosen on their own phenotype, or whole families can be kept or dropped on the basis of the family mean. Within-family selection keeps the best fish from each of several families.
• Progeny testing for recessives: a fish showing the dominant phenotype is mated to a test fish that is homozygous recessive. Any recessive offspring reveal the parent as a carrier.
• Health and vigour: since inbreeding depression lowers growth, fecundity and survival, these belong among the criteria alongside appearance.
• Breadth: each act of selection is a bottleneck, so keeping breeders from several families slows the loss of genetic variation.

Culling and rehoming

In breeding terminology, culling means taking a fish out of the breeding programme. It does not specify what then happens to the fish. Options for non-selected fish include keeping them in separate grow-out tanks, passing them on to other keepers, or humane killing, and views on which option is acceptable differ. Whatever the choice, the fish should never be released into natural waters. Escaped farmed fish can compete with native populations or interbreed with them and leave the local population less fit. Humane killing should follow veterinary guidance and local law.

Generation planning

Generation length sets the pace. Female guppies generally give birth about once a month. In the wild, the guppy generation interval is around 100 days in complex fish communities and 180–200 days in small streams (Chang et al. 2024). In the guppy selection study above, traits were measured when the fish were six months old.

1. Define the goal and how it will be scored, for example a measured area or a yes/no colour class.
2. Found the line from as many unrelated breeders as possible and keep the sex ratio near 1:1, because a skewed ratio and a few dominant families shrink the effective breeding number.
3. Split breeders into two or more groups and rotate matings between groups each generation, which prevents brother-sister matings and slows inbreeding.
4. Record parents, brood size, survival and scores for every generation.
5. If fertility or survival falls, relax selection for one generation by crossing selected fish with good fish from an unrelated line.

Avoiding welfare-compromising traits

• Balloon body: balloon forms of mollies, cichlids, gouramis and tetras have squat bodies with severely distended abdomens. A Practical Fishkeeping editorial reports that many such fish struggle with buoyancy and swim head-up or tail-up, show spinal curvature and gastrointestinal problems, and often live less than half their expected lifespan.
• Long and heavy fins: male Siamese fighting fish (Betta splendens), bred for large, elaborate median fins, jumped for a shorter time, less high and with lower top speed than females, and than the same males after their fins were trimmed (Jamison et al. 2017, conference abstract).
• Absent dorsal fin: goldfish lacking a dorsal fin swim more slowly, accelerate more slowly and less efficiently, and tend to roll (Blake et al. 2009, summarised by Science for Animal Welfare).
• Eye modifications: in bubble-eye goldfish, the fluid-filled sacs below the eyes cause partial blindness and are easily damaged, and lifespan is about half that of normally shaped goldfish. Celestial-eye goldfish have upturned eyes with a retina that progressively degenerates. Science for Animal Welfare recommends that neither breed be perpetuated.
• Loss of natural breeding: some highly selected goldfish can no longer breed naturally because of their altered body shape.

These problems can be eliminated by not breeding from fish that show such traits. A breeding goal that leaves swimming, vision, buoyancy, feeding and natural reproduction intact avoids most of them. Some breed organisations set this as their aim: the International Betta Congress, founded in 1967, set out to breed healthier varieties with more symmetrical fins and bodies, with an emphasis on animal welfare.