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Biology

Tampering with the food web: what do changes in jellyfish and menhaden populations mean for the ecosystem?

Article: Robinson, K.L., J.J. Ruzicka, F. J., Hernandez, W.M. Graham, M.B. Decker, R.D. Brodeur, and M. Sutor. 2015. Evaluating energy flows through jellyfish and gulf menhaden (Brevoortia patronus) and the effects of fishing on the northern Gulf of Mexico ecosystem. – ICES Journal of Marine Science, doi: 10.1093/icesjms/fsv088

Background

We’ve all grown up learning “who eats who” in an ecosystem using diagrams connecting animals’ predators and prey, commonly referred to as food webs. This is just one of many questions that food web dynamics help us understand. An often overlooked question is “who competes with who” for food and space. By identifying each species’ predators and prey, one can start thinking about how changes in a single organism’s population may cascade, directly or indirectly, to other organisms. Understanding these interactions is particularly important when assessing how ecosystems will change as species are commercially fished, impacted by increased temperatures, or respond to other habitat alterations. Some of the most important groups in marine food webs are forage fish. Forage fish are small, schooling fish that are critical in transferring energy from low to high trophic levels; they often serve as the link between planktonic primary producers and organisms higher in the food web (e.g., tunas, marine mammals and seabirds). For many years, jellyfish have been overlooked in food webs, and were previously considered “trophic dead ends”. However, in reality, they play a very similar role to forage fish in the food web, with the same prey types and large predators. The trophic similarities between forage fish and jellyfish have lead scientists to further investigate jellyfish’s role in the food web, and how it compares to that of forage fish. The major question scientists have is: what do changes in forage fish or jellyfish populations mean for the rest of the ecosystem?

Methods

A recent study by Robinson and colleagues addressed this question by modeling the species’ interactions in the northern Gulf of Mexico food web. The modeling approach tests how starting biomass and production change when varying rates describing ecological processes (e.g., predation, energy transfer efficiency) and/or human interactions with species (i.e., fishing). The model includes all types of species in the ecosystem, from small critters like plankton to large organisms like whales, which allows scientists to account for all aspects and energy flows in the food web. The species are either defined as their own group, or grouped with other species that represent a similar ecological function (called a functional group). With information on functional groups’ biomass, production, and system rates, the authors were able to test how changes in these model inputs could change the entire ecosystem. Specifically, these scientists estimated food web production under varying conditions for jellyfish (Auriela sp. and Chrysaora sp., Figure 1) and the forage fish Gulf menhaden (Brevoortia patronus, Figure 2). Gulf menhaden were chosen as the forage fish species to model because they support the largest forage fish fishery in the United States (second largest U.S. fishery), and have a strong seasonal overlap with jellyfish blooms (spring through summer). Four scenarios were tested with the food web model:

220px-Aurelia_aurita_1

Figure 1. Scyphozoan jellyfish, Aurelia sp.

Figure 2. Gulf menhaden (Brevoortia patrnous)

(1) increase large jellyfish consumption of plankton prey by 50%,

(2) remove menhaden by increasing forage fish fishing pressure by 50% and decreasing the menhaden population by 59%,

(3) reduce forage fish fishing by 50% and  increase menhaden stock by 41%,

(4) close of all fisheries, increasing populations for fish groups

Results

Scenario 1: Under the first scenario, the authors found that total ecosystem productivity declined, with large decreases for baleen whales, cephalopods (e.g., squid) and forage fish (including menhaden). The increased jellyfish feeding highlights that jellyfish outcompete small crustaceans, baleen whales, and small pelagic fish for critical plankton prey.

Scenario 2: In the second scenario, the authors also found that increasing menhaden fishing and decreasing menhaden biomass caused over a 10% decrease in total ecosystem productivity, particularly in fish and seabirds, and an increase in jellyfish production (Figure 3). This outcome reflects the significance of competition between forage fish and jellyfish, and identifies menhaden as a major, more effective pathway than jellyfish in transferring trophic energy from the bottom of this food web to the top. The resulting increase in overall ecosystem productivity in scenario two prompts the question: are menhaden more valuable left alone in the ecosystem and not fished? Under this scenario, some menhaden predators compensated for reduced menhaden and maintained their production levels by eating other plankton-feeding fish (e.g. mullet, anchovies, herrings).

Figure 3.  Scenario 2: Less menhaden means less seabird and fish production, and more jellyfish production.

Figure 3. Scenario 2: Less menhaden means less seabird and fish production, and more jellyfish production.

Scenario 3: Reducing fisheries under scenario three had minimal changes for the ecosystem overall, with many functional groups changing very little. Marine birds, small fish and predatory fish groups increased a little, with fishery productivity decreasing by 28% (as expected with reduced fishing effort).

Scenario 4: In scenario four, menhaden, select large predator fish, and seabirds’ productivities increased by 81, 15 and 16 percent, respectively. Jellyfish and other forage fish productivities, on the other hand, decreased with the fishery closures.

Significance

This work highlights jellyfish’s significance in regulating ecosystem productivity via competition with forage fish for prey. Additionally, potential increases in jellyfish highlight the negative consequences for the ecosystem, with fewer menhaden available for both predators and commercial fisheries, and less overall ecosystem production. What makes this scenario particularly worrisome is that this concern could become a reality. As sea temperatures have risen in recent years, scientists have also found jellyfish populations increasing. With continued temperature increases and thus more jellyfish, this study allows for us to understand the potential impacts of climate change on an ecosystem level.

Additionally, the authors show that ecosystem modeling helps us understand the species interactions, and identify potential changes, those both predicted and unforeseen! This work also provides a base for further discussion on weighing the balances between ecosystem productivity with commercial fishing harvest. Identifying and understanding the relationships between marine species and their environment are critical for making informed decisions on fishing regulations and preparing for changes in the Earth’s climate. By continuing to identify how ecosystem components interact and how systems change under varying conditions, we will be able to make more informed decisions to preserve fish populations and ecosystems moving into the future.

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