Finding the “golden stocking density”: A balance between fish welfare and farmers' perspectives

Abstract

The effects of stocking density on fish welfare are complex and involve many interacting parameters. This complex relationship between fish welfare, stocking density and influencing factors make it challenging to define a specific optimal (“golden”) stocking density. Indeed, previously published recommendations on stocking density for different species of aquaculture interest are incredibly variable even at the same life stage, and can also vary widely within a rearing unit. Production density can be estimated quite accurately if the farmer has good biomass control and a known water volume, but it is difficult to set minimum and maximum stocking density levels that will protect welfare. However, there is little doubt that stocking densities that are too low or too high can have negative impacts on welfare and/or production. Here, we propose how to select density on captive fish and monitor its potential effects integrating 1) solid welfare assessment based on operational welfare indicators and 2) good management practices. Regulation directly limiting stocking density is likely to be unworkable and ineffective, and a more rational option might be to prescribe acceptable levels of different welfare indicators (e.g., water quality, health, nutritional condition and behavioral indicators), which together with a positive economic balance of the company, allow to estimate the most suitable range of fish density for each particular species, life-stage and production systems.

The value of regulating stocking densities in aquaculture must not be dismissed: a reply to Saraiva et al. 2022

“…In their mini review, Saraiva et al ( 1 ) claim that “From fish farmers to certifiers and even animal advocacy groups, all parties…[are] searching for the “golden stocking density” (p2) and that this search is biased depending on the stakeholder's agenda. There are, of course, many perspectives to consider here, but from an advocacy perspective, advocates are not seeking regulation of a “golden stocking density” but are rather seeking stocking density limits that reduce many welfare risks, including chronic welfare issues, and that allow for a good life for farmed fish ( 2 ).…”

“…In their paper, the authors ( 1 ) discuss the three approaches to animal welfare, namely, the feelings-based approach, which considers the animal's mental state; the function-based approach, which is concerned with the animal's health and functioning; and the nature-based approach, which refers to the need for a natural environment and the expression of natural behavior ( 23 , 24 ). Then, they rightly say that integrating the three can help operationalise the animal welfare concept.…”

“…Saraiva et al ( 1 ) argue that “Legislation directly limiting stocking density is likely to be unworkable…” (p. 4). We disagree as there are already relevant, proven track records for using stocking densities in aquatic- and terrestrial-farmed animal welfare legislation.…”

“…In their mini-review, Saraiva et al ( 1 ) suggested that in place of stocking density limits, “…a more practical option might be to prescribe acceptable levels of different welfare indicators (e.g., water quality, health, nutritional condition and behavioral indicators),….” A similar argument was made by the New Zealand (NZ) government when they decided against lowering broiler stocking densities, as they argued that animal welfare should be addressed more holistically, using a combination of other factors, including management skills and environmental issues. Sankoff ( 51 ) reviewed NZ's Animal Welfare Act 1999 and claimed this decision to be “…entirely unsatisfactory” (p. 23), and we agree.…”

Social behaviors as welfare indicators in teleost fish

“…Many of these social behavioral traits are drastically affected when stocking densities are too low or too high, impacting directly on fish welfare. Since stocking density is a very important topic in aquaculture, an integrative approach should consider fish welfare and farmer’s perspectives by integrating solid welfare assessment (operational welfare indicators) with good management practices ( 37 ). Moreover, considering that social behavior can be affected by the environmental enrichment and that in most cases this focuses on structural enrichment, it is important to emphasize that many other strategies are essential to improve farmed fish welfare, such as sensorial (visual, auditory, and chemical stimuli), occupational enrichment (aiming to introduce diverse challenges to the rearing environment to prevent monotony), social and dietary enrichment [reviewed in ( 38 )].…”