Notre Dame graduate student Peter Levi takes a break from his research on Pacific salmon to tell us about their impacts as well as the impacts of their Atlantic brethren.
Atlantic salmon (Salmo salar) are a popular dinner item from coast-to-coast. But the number of these fish in the Pacific Ocean may soon outnumber those in the Atlantic Ocean! That may sound preposterous…or it may simply sound like another case of invasion.
Atlantic salmon are a key aquaculture species, especially off the coast of South America. Paradoxically, Atlantic salmon are endangered in the Atlantic Ocean, so farming them may be the only way to get them to your plate. However, these farms come at a cost to native organisms in the Pacific Ocean, including Pacific salmon.
Salmon farms are often densely populated, which may lead to increased diseases and pathogens that could spread to native salmon. Additionally, escaped Atlantic salmon may bully Pacific salmon out of their native spawning sites. Finally, although it appears Atlantic-Pacific salmon hybrids are difficult to produce, the fear of genetic interactions between salmon species still looms, especially among locally threatened and endangered Pacific salmon populations.
Why does an Atlantic salmon invasion matter? Pacific salmon play a key role in their native ecosystems. When juvenile salmon migrate to the ocean from the stream or lake where they were born, they are often no larger than your finger. Salmon feed and grow in the ocean for one to six years, depending on the species, putting on many pounds and growing to the length of your arm, but with much more mass. Pacific salmon only make one return migration to their home stream or lake, dying after spawning and protecting their nests for as long as they still have energy.
Look closely to see hundreds of pink salmon in Maybeso Creek, Alaska. Photo Credit: Peter Levi
Salmon bring literally tons of nitrogen, phosphorus, and carbon with them, which fertilizes plants and animals in and around these freshwater ecosystems, from algae and fish to brown bears and trees (also recall that introduced salmon runs in the Great Lakes can also represent an influx of contaminants). In contrast to enriching freshwaters, salmon runs can also be a destructive force via upstream migration and digging of nests. Recent research has shown that stream structure is an important determinant of whether salmon have a net enrichment or disturbance effect on their natal surroundings. In contrast to native Pacific Salmon, Atlantic salmon will spawn in multiple years before dying, likely having a greater disturbance than enrichment effect since they take their nutrients back to the ocean with them after spawning. Therefore, if they invade and replace Pacific salmon, the natural ecological dyanmics will be severely disrupted.
With such important ecosystem impacts, it is worth giving some extra thought to sustainable consumption the next time you are planning a salmon meal, whether the Atlantic or Pacific variety.
Far more in the spirit of invasive species awareness than direct management, I bought tilapia for today’s recipe. And in this confession lies the crux of eating invasive species.
Finished Almond Tilapia. Photo A. Deines
Should consumer demand for edible invasives ever reach the fervor required to drive harmful populations to extinction, that same economic force would provide incentives to farm and/or further spread the species in new locations. Much touted as a sustainable alternative seafood, tilapia is a good example of market demand for an invasive species driving it’s invasion.
Without good legislation combating the spread of harmful species like tilapias, invasions will continue to be substantial economic externalities for meals like this. Good legislation, however, begins with awareness. So in that spirit…
Recipe: Almond tilapia
Almond tilapia ingredients. Photo A. Deines
2 tilapia fillets, about 150g (5 oz) each
1/4 cup olive oil
1/4 cup slivered almonds
1/2 cup whole wheat flour
salt and pepper to taste
Heat olive oil in a large skillet at a medium temperature. While the oil heats, lightly coat the tilapia fillets in whole wheat flour. Briefly saute the slivered almonds, then lay the floured fillets directly on top of the almonds (see picture below). Drizzle lemon juice and some salt onto the fish and cover with a lid.
Tilapia fillets cooking over almonds. Photo A. Deines
Cook for 3-5 min based on thickness of the fillet. The almond slivers should begin to turn brown. Using a wide flat spatula, flip the fillets over while being sure to hold the almonds together with the fish. Cover the skillet again and cook another 3-5 mins, until the thickest portion of the fillet flakes when probed with a fork. Remove the fillets onto serving plates, and drizzle with some more lemon juice.
Finished Almond Tilapia. Photo A. Deines
I sprinkled some parsley onto the finished product for show, and added some salt and pepper to taste. I recommend a steam or stir-fry vegetable side dish, which seasonally offers great potential for it’s own invasivory. White wine is traditional with a white fish, but I had a Bota Box malbec, which was perfect with the earthy tones of the vegetables and the dark roasted almonds.
It’s the middle of winter here in Indiana, making hunting and gathering difficult. So to kick off a week of tilapia, this is one of my favorite traditional Zambian dishes from my time in the Peace Corps and more recently for my PhD research. It’s salty and delicious, but not typically how Americans cook fish.
Pan Fried Tilapia, Traditional Zambian Style
Tilapias are a diverse group of African cichlids. In the last 60 years, many of these species have been moved around the world for stocking, mosquito and aquatic weed control, and aquaculture. The most common tilapia on menus and ice at the grocer is Nile tilapia, Oreochromis niloticus. Increasingly common at meal time, under many circumstances tilapias are considered highly invasive. Native only to Africa, it’s actually very likely that the tilapia Americans consume are farmed in Central America, or China.
Tilapia at the local grocery. Photo credit A. Deines
Indeed, the global market for tilapia is booming. It shouldn’t be surprising then that tilapias have escaped these farms and established feral populations in many countries far outside of their native ranges.
The global tilapia market has also caused Nile tilapia to be translocated within Africa, to regions where it is not native, but where related species do occur naturally. For example, the Three-spot tilapia, Oreochromis andersonii, the focus of my PhD research, which I blatantly plug:
While I had to go to Zambia to collect tilapia, you can get tilapia at the grocery store, though the economics of this actually promotes tilapia invasion. As an invasivore it’s much better to catch it yourself if you live in some of the many locations where it’s known to have feral populations. For instance, the Mozambique tilapia is essentially the only fish surviving in California’s Salton Sea. Tomorrow we’ll be rolling-out an interactive map to help invasivores find their query.
Tilapia can be caught with a fishing pole using worms but I’ve also had success with spinners. Cast-nets can be effective in areas where tilapias build nests, which are easy to spot. A tilapia nest looks like a mud volcano or caldera sometimes up to meter across. In Zambia, the most effective (but illegal) method of harvest is known as “kutumpula”, where fishermen beat the water to scare the fish into gillnets. Be sure to check into and abide by local fishery rules when you harvest.
Recipe: Traditional Zambian Pan Fried Tilapia
Frying small tilapia in Zambia. Photo Credit A. Deines
1 whole tilapia, about 100-300g (~1/2lbs)
1 cup vegetable oil
1 cup corn flour
After gutting and scaling the tilapia, make two or three shallow cuts into the flanks of the fish running diagonally down from the base of the dorsal fin towards the belly of the fish (see picture for example). Sprinkle some salt into the cuts, and then lightly coat the tilapia in corn flour. Heat about ½ cm of oil in a frying pan at medium heat until it crackles when a small amount of water is dripped in.
Carefully place the fish into the oil. Fry for up to 10 minutes on the first side, carefully watching the heat, so as not to burn. Yes, this seems a very long time to fry a fish, until the outer layer of flesh is brown and crispy. While the fish is frying on the first side, dice the onion and tomato. Soon after turning the fish, slide the tomato and onion into the oil, alongside the fish and continue frying. Remove the fish from the oil when it is brown and crispy on the second side.
Continue to cook the tomato “soup” and add about ½ tbs salt. Stirring often with a wooden spoon, cook another 5 minutes, homogenizing the soup by gentle mashing. Pour the soup over the fish, and serve. Nshima and “rape”-the leaves of the canola oil plant, Brassica, also invasive in the US- make excellent accompaniments. Though in Zambian culture, it’s always the fish and vegetables that accompany the staple Nshima.