The main climate stressors that driving changes in global fish populations include, rising ocean temperatures, lower pH, and lowered oxygen levels (Crozier and Hutchens, 2014). According to the IPCC, a global sea temperature increase above 1.5 degrees Celsius will force some fish and other marine organisms north to higher latitudes (Cooley et at., 2021). Further, the range of most marine species is shifting poleward at around 59.2 kilometers per decade (IPCC, 2021). Since 1850 ocean surface temperatures have increased by around 0.88 degrees Celsius (Fox-Kemper, et.al, 2021).

This is a map that shows the range of striped bass across North America.

  • Peterson Field Guide to Freshwater Fishes, 2011. Second Edition, by Lawrence M. Page (Author), Brooks M. Burr (Author), Eugene C. Beckham (Illustrator) ISBN 9780547242064

Low pH Levels in the Ocean

Humans have added so much CO2 to the atmosphere, that the oceans are now absorbing more CO2 than they can process. Due to the increased uptake of CO2 by the oceans around the globe, sea water is acidifying at an increasing rate. When CO2 enters the ocean, it is dissolved, and reacts with the water, producing excess hydrogen ions, and bicarbonate, while decreasing carbonate concentration in the ocean (Canadell et al., 2019). Acidification has many impacts on ocean ecosystems, these include causing biodiversity loss, shifting food webs, and weakening calcifying organisms (Ullah, 2018). Since the 1980’s it is very likely the pH of open ocean has been decreasing by about 0.017 – 0.027 pH units per decade (Bindoff et al., 2019).

Ocean Warming

Most of the warming in the ocean can be attributed to anthropogenic changes in the Earth’s radiative properties (Canadell et al., 2019). The increase in greenhouse gas emissions has led to an energy imbalance in the atmosphere.

93% of the energy that would normally leave the atmosphere is being absorbed back into the ocean. (Johnson and Leyman, 2020).

This energy works to warm the oceans average temperature. The warming of the oceans is virtually certain to persist throughout the 21st Century. The heat content of the world’s oceans has been increasing since 1970, and the slow circulation of deep ocean currents will keep that heat circling the globe warming the waters as it travels (Fox-Kemper, et.al, 2021).

Decreasing Oxygen Levels in the Ocean

The decrease in oxygen levels in the ocean, or ocean deoxidization is the direct result of warming temperatures (IPPC, 2022, Chapter 3). Oxygenation of the ocean is linked with constraints on productivity, biodiversity, and biogeochemical cycles (Breitburg et al., 2018). When the dissolved oxygen content is less than two milligrams per liter, it is known as hypoxia which can be deadly to some organisms (Maryland.gov, 2022). The combination of acidification and deoxidization is becoming more common in coastal systems, and the impacts from the combination of these factors require further research (Gobler and Bauman, 2016).

Striped Bass Exposure to Climate Change Hazards

Exposure to Warming Waters

Striped bass are anadromous fish, meaning they migrate to estuaries and fresh water sources along the east coast, before returning to the Atlantic Ocean. Striped bass spawn in estuaries such as the Hudson River Estuary and The Chesapeake Bay (Wojtusik et al., 2022). In response to warming ocean temperatures, the recruitment of fish species in the north-east Atlantic, has been declining since 1970 (Poloczanska et al., 2019). The migration of striped bass takes them out of coastal estuaries, as far north as Canada, and south to the Gulf of Mexico (Wojtusik et al., 2022). Specifically in the Hudson River Estuary, biodiversity loss and habitat destruction have worked to destabilize this ecosystem (O’Connor et al., 2012). The shallower estuaries in which striped bass spawn, have been identified as areas of increased water temperature warming (Oczkowski et al., 2015).

Striped Bass Exposure to Decreased pH Levels in the Ocean

Coastal ecosystems face several factors that increase acidification compared with open ocean systems. Discharge of acidified riverine water, acid deposition, sea ice melting, and the low alkalinity of coastal zones result in the lowering of the buffering capacity against CO2 in these coastal areas (Gobler and Bauman, 2016).

Peer and Miller. (2012). [Figure 1 - Map of the Chesapeake Bay and surrounding region; the Striped Bass spawning grounds in the Upper Bay and Potomac River are highlighted in white. Black circles represent stations from which environmental data used in the analyses were obtained (MCB = Marine Corps Base; NAS = Naval Air Station)]

Vulnerability of Striped Bass to Climate Change

Warming Impacts on Striped Bass Range and Migrations

John Lyman, Joint Institute for Marine and Atmospheric Research.(2020). NOAA. https://www.pmel.noaa.gov/news-story/ocean-warming-trends-dwarf-cooling…

Water temperature increases have already begun shifting the migrations of North Atlantic fish species northward. Less is known about the range of striped bass migrations, but it is fair to assume the range of striped bass is shifting north as well (Nack et at., 2019). Although some fish populations have been responding positively to increased temperatures, this positive trend cannot be expected to continue as temperatures surpass optimal growth conditions (Free, et.,al 2019). Spring water temperatures in the Chesapeake Bay have shown the greatest warming, which coincides with the annual striped bass spawn in the spring months (Oczkowski et al., 2015). Spring water temperatures indicate when female striped Bass spawn and lay eggs. As ocean temperatures increase the peak migration time for female striped Bass will become earlier. The migration timing for female striped bass has been projects to occur three days earlier for every one-degree Celsius the water temperature rises (Miller et at., 2014). This exposes these vulnerable female fish to higher fishing mortality rates (Miller et at., 2014).

Vulnerability to Acidification and Hypoxia

The combined effects of warming and acidification have been shown to decrease the transfer of energy between trophic levels, this loss of energy could lead to a collapse of marine food chains from the bottom up (Ullah, 2018). Due to its correlation with temperature, the frequency, size and duration of hypoxic zones are expected to increase as climate change warms the globe (Gobler and Baunman, 2016). Juvenile striped bass using estuaries as nursing grounds may find themselves in hypoxic zones. The survival of these fish is dependent on their hypoxia tolerance, which can vary between specific fish in a species. If the hypoxia tolerance of a juvenile striped bass is low enough, they will eventually die from the low levels of oxygen (Nelson, et.al, 2019). It has been shown that increased acidification can make fish species more vulnerable to hypoxia, and cause mortality at higher dissolved oxygen levels then the past (Miller et al., 2016).

Spawn and Growth Vulnerability

Fish species that have faced overfishing in the past have been shown to be more susceptible to warming water temperatures (Free et al., 2019). Warmer ocean temperatures have also been linked to an increase in the duration of striped bass spawning. In the Hudson River Estuary, the duration of spawn time increased by five days in both the upper and lower spawning grounds of the Hudson River Estuary (Pan et.al, 2023). Models are constrained to the notion that warming either has a positive impact or negative impact on fish populations, but this is not the case for all fish. For most fish species there is an optimal growth temperature, and productivity will increase until this temperature, then decline once temperatures exceed the threshold (Free et al., 2019). The increased ocean warming has shifted zooplankton distribution north, as this continues it could pose a threat to productivity of fish species that use these organisms as prey (Peck and Pinnegar, 2019).

Pacific Southwest Region USFWS. (2017). WikimediaCommons. https://commons.wikimedia.org/wiki/File:Striped_bass_(39492037735).jpg

Adaptation Strategies and Resilience in Striped Bass in Response to Climate Change

 

Various adaptation strategies have been identified for warming water temperatures in the North Atlantic. In response to changes in reproduction, growth and mortality associating with warming waters, the following adaptation strategies have been identified and can be applied to striped bass. (Paukert et al., 2021)

  • Leveraging tourism to help promote citizen science
  • Maintaining and creating diverse and large habitats to broaden the gene pool

Further, as fish species shift geographically, implementing protected areas, and using regulations to increase the harvest of fish which are adapting better to warming waters. (Paukert et al., 2021) Scientists in Florida have studied the temperature tolerance of different regional striped bass populations and found the Florida population more tolerant to warming waters. (Kenter & Berlinsky, 2022)

Striped bass have been identified to avoid hypoxic areas but will tolerate higher temperatures if there is sufficient food (Kenter & Berlinsky, 2022).

The best but also most challenging strategy to combat ocean acidification is reducing greenhouse gas emissions worldwide (Paukert et al., 2021).

 

Final Remarks

 

In all climate change impacts such as rising temperature, increasing acidification, and less oxygen in the ocean has impacted marine fish species in various ways. It is often a combination of these impacts that has the largest effect on fish species. For stiped bass, warming ocean temperatures have been seen to impact timing of migration and duration of spawning. The combination of acidification along with warming temperatures impact the bottom levels of the food web, by decreasing energy transfer between organisms. In areas of low oxygen, it requires more energy from species to process the oxygen in the ocean. This leads to biodiversity and productivity of fish species decreasing. Despite these hazards there exist possibly adaptation strategies which can help striped bass endure climate change.

A word about the author

Mathias Heinz is a combined Mathematics and Environmental Studies major who graduated St. Lawrence University in 2024. Growing up on Block Island, Rhode Island, Mathias was always around the ocean and marine species, and had continued interest in these areas during his college studies. This webpage was created for ENVS 329 Adapting to Climate Change for Professor Jon Rosales in Spring 2023.

Photo Credit: Ryan Krugman

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