Thanks for this, I'm trying to at least get a couple of numbers to get in the right ballpark to help people calibrate. Epistemic status: uncertain, a couple of hours of research. Takeaway: Probably billions of fish killed.

To get a higher end of the estimate of total fish killed as a result of mosquito net fishing, the appendix of the paper you link to gives an example of a single large-scale fishery in Madagascar where 68.7 million fish were killed in 2018-2019 by 75 families. 

Unless I'm misreading, this looks like 80-90% of the total fish numbers were caught through mosquito net trawler (27.7% by weight, but much smaller fish on average, so a higher number).

This other paper in Mozambique provides another example - they two fishers catch around 35kg of (mostly tiny, < 20 mm, so 1-10g?) fish a day, so I guess around 10,000 fish a day? Over 2 million a year (so similar ballpark to Madagascar). 

These are the locations at which mosquito net fishing had been observed in this study (data mostly from 2015), with a rapidly rising trajectory.   

I can't seem to find any better data on the scale of fishing at all of these locations, so I have no idea whether it's closer to 50 or 10000 locations where mosquito nets are used for fishing to a similar extent to those in these articles, but 25 million fish * 50 locations would be 1.25 billion fish.  

I think it's safe to assume that at least 1 billion fish, possibly orders of magnitude higher, are killed yearly through mosquito net fishing, most of whom wouldn't be killed if it weren't for the distribution of mosquito nets (due to the small mesh sizes). 

Of course, to make a reasonable welfare calculation, you shouldn't just look at the number of animals killed and how painful their deaths must have been. You also have to consider how long these fish would have lived for counterfactually, the quality of their lives and suffering involved in their counterfactual deaths, as well as broader ecosystem effects etc. 

But @Dylan Matthews's claim that "There is little research on what fishing with these nets actually does to fish or people — but also little reason to think the magnitudes of these effects are remotely near the number of lives saved by nets" strikes me as indefensible.  

Causing billions of fish to experience a painful death is not just a rounding error, and it may well be worth reevaluating whether bednets have had a positive net impact or not. 

Thanks for the post Rachel! I admire you looking into the topic. 

However, one to add is that AMF monitors net usage uniquely stringently and this likely leads to far fewer people using the nets for fishing.  (or at least they did until 2015)

From a 2015 article:
However, AMF takes a number of steps to stop misuse. We believe that each of them has contributed to the success of our distributions.

1. Malaria education is key. We work closely with local health officials, the front line health teams and the village and community leaders, to ensure the community is fully aware of the connection between sleeping under intact nets and preventing malaria.
   
    
2. AMF uses extremely rigorous pre-distribution surveys to determine precise net needs, followed by independent checks when the nets are given out to ensure no nets are diverted away from households that need them.
   
    
3. AMF conducts post-distribution check-ups to ensure nets are being used as intended every 6-months during the 3 years following a distribution. People are informed that these checks will be made by random selection, and via unnannounced visits. This gives us a data-driven view of where the nets are and whether they are being used properly. We publish all the data we collect: example here. This type of follow up work is currently unique to AMF, although we hope the practice will become more widespread where it is feasible.

At AMF, the extensive data we have collected verifies that the number of nets we have distributed that are used for fishing is immaterial.
https://www.againstmalaria.com/NewsItem.aspx?newsitem=Net-use-and-the-importance-of-data-driven-distributions-and-monitoring

Given that AMF is the main bednet distributor that EAs give to (Malaria Consortium distributes medicine), I think the negative impact of EA-influenced bednet distribution is lower than one might assume from your post!

'I think this is a relatively small update, unlikely to change my mind about the overall positive benefit of free mosquito net distribution'

Have you actually run a back of the envelope calculation on this? I'm kind of worried we are all massively biased on this topic because it would be so embarrassing for EA if mosquito net distribution turned out to be net negative. 

Executive summary: There is some evidence that using mosquito nets for fishing harms fish populations more than previously thought, but likely not enough to outweigh the benefits of distributing free mosquito nets. 

Key points:

1. Mosquito nets can catch juvenile fish, harming future fish populations.
2. Some surveys and studies link mosquito net fishing to fisheries decline, but the full global impact is unclear.
3. Mosquito net fishing seems widespread where nets are distributed and fishing resources exist.
4. The overall benefits of mosquito net distribution probably still outweigh the fishing harms.
5. Further mitigation work could come from outside EA, while preserving net access.

This comment was auto-generated by the EA Forum Team. Feel free to point out issues with this summary by replying to the comment, and contact us if you have feedback.

Could nets be made such that their mesh size increases when they get soaking wet?

PS: Are the insecticides also a problem? Are there biodegradable insecticides?