Appendix: huge numbers of juveniles raised for an unknown reason
I suspect they’re raised as feed for other farmed fish (and maybe other farmed aquatic species). Maybe they could also be released into wild fisheries as feed for wild-caught aquatic animals.
Aquaculture production was 28.02 million tonnes in 2013, and the corresponding production of artificially propagated fry was 1 914.3 billion.
And they have a figure:
They have a section “6.4.4.2 Fry Production of Prey Fish as Feed for Predatory Fish”. They specifically discuss mud carp fry as feed for mandarin fish (Siniperca chuatsi). They write:
6.4.4.2.3 The Relationship Between the Production of Mud Carp Fry and Mandarin Fish
Mud carp, is the favorite prey fish of mandarin fish. The production of mandarin fish has increased in relation to the growth of mud carp culture. The production of mud carp per growth cycle is about 7500 kg/ha, while mandarin fish was about 6000–7500 kg/ha, and feed coefficient was about 1:3–4. When the feed coefficient is 3.5, the production of mandarin fish was 284 780 tonnes in 2013, and required a prey fish production of mud carp of about 996 730 tonnes. Almost all prey for mandarin fish is provided through artificial propagation. The production of mandarin fish has increased over the years, and is significantly positively correlated with fry availability (Figure 6.4.9) (Pearson correlation = 0.70, P < 0.01) (China Fishery Statistical Yearbook 1996–2014). As a high‐quality food for mandarin fish, the variation in production of mud carp is directly related to the aquaculture scale of mandarin fish, as shown from the example in Guangdong Province (Figure 6.4.10). Here again, there is a significant linear correlation between the production of mandarin fish and mud carp (y = 0.348x – 48057, R2 = 0.765, P < 0.05) (Yao 1999).
(Though looking at the figures 6.4.9 and 6.4.10, fry production and mandarin fish production don’t look very closely related, and it could just be all aquaculture going up.)
Well fuck, I guess this probably explains it. Yao & Li, 2018:
Mandarin fish have unusual feeding habits. The fish only eat live fish and shrimps, and do not consume dead prey or artificial diets during all lifecycle stages (Chiang 1959; Li et al. 2014a; Yao and Liang 2015). In nature it is completely carnivorous, and has been found to capture live fry of other fish species from the first feeding stages (Chiang 1959).
Also makes substitutes for fish fry not very promising; they’d probably also have to be other animals. But maybe we could find some that matter much less per kg.
Otherwise, we’d probably just want to reduce mandarin fish production, which could be hard to target specifically, especially being in China.
According to data from the China Fishery Statistical Yearbook, the fry number of freshwater fish increased from 59.51 billion in 1981 to 1.252 trillion in 2019, while the fry number of marine fish increased from 167 million in 1996 to 11.44 billion in 2019 [6,7].
I suspect they’re raised as feed for other farmed fish (and maybe other farmed aquatic species). Maybe they could also be released into wild fisheries as feed for wild-caught aquatic animals.
From Li and Xia, 2018, first some updated numbers:
And they have a figure:
They have a section “6.4.4.2 Fry Production of Prey Fish as Feed for Predatory Fish”. They specifically discuss mud carp fry as feed for mandarin fish (Siniperca chuatsi). They write:
(Though looking at the figures 6.4.9 and 6.4.10, fry production and mandarin fish production don’t look very closely related, and it could just be all aquaculture going up.)
https://thefishsite.com/articles/cultured-aquatic-species-mandarin-fish, https://www.fao.org/fishery/affris/species-profiles/mandarin-fish/faqs/en/, and https://www.fao.org/fishery/affris/species-profiles/mandarin-fish/natural-food-and-feeding-habits/en/ also discuss fish fry fed live to mandarin fish.
Well fuck, I guess this probably explains it. Yao & Li, 2018:
Also makes substitutes for fish fry not very promising; they’d probably also have to be other animals. But maybe we could find some that matter much less per kg.
Otherwise, we’d probably just want to reduce mandarin fish production, which could be hard to target specifically, especially being in China.
Some different fry numbers in Hu et al., 2021:
I’m writing a quick piece on the scale, in case you (or anyone else) is interested in giving feedback before I post it (probably next week).