No ocean for old fish / by Francisco Blaha

A new paper by Lewis A.K. Barnett, Trevor Branch and other researchers from the School of Aquatic and Fishery Sciences of the University of Washington titled Old-Growth Fishes Become Scarce under Fishing, have found that for dozens of fish populations around the globe, old fish are greatly depleted, mainly because of fishing pressure, hence are increasingly missing in many populations around the world. 

Old fish become scarce under fishing.

Old fish become scarce under fishing.

I got to this paper via a ScienceDaily line that said: Like old-growth trees in a forest, old fish in the ocean play important roles in the diversity and stability of marine ecosystems. Critically, the longer a fish is allowed to live, the more likely it is to successfully reproduce over the course of its lifetime, which is particularly important in variable environmental conditions. 

The paper abstract quotes: 

Researchers have long recognized the importance of ecological differences at the species level in structuring natural communities yet until recently have often overlooked the influence of intraspecific trait variation, which can profoundly alter community dynamics. Human extraction of living resources can reduce intraspecific trait variation by, for example, causing truncation of age and size structure of populations, where numbers of older individuals decline far more with exploitation than younger individuals. Age truncation can negatively affect population and community stability, increasing variability in population and community biomass, reducing productivity and life-history diversity in traits such as the spatial and temporal pattern of reproduction and migration. Here, we quantified the extent of age truncation in 63 fished populations across five ocean regions, as measured by how much the proportions of fish in the oldest age groups declined over time.
The proportion of individuals in the oldest age classes decreased significantly in 79% to 97% of populations (compared to historical or unfished values, respectively), and the magnitude of decline was greater than 90% in 32% to 41% of populations. The pervasiveness and intensity of age truncation indicate that fishing is likely reducing the stability of many marine communities. Our findings suggest that more emphasis should be given to management measures that reduce the impact of fishing on age truncation, including no-take areas, slot limits that prohibit fishing on all except a narrow range of fish sizes, and rotational harvesting.

The ScienceDaily  article provides some good explanations:

In forestry, a tree farm with only 20-year-old trees may be healthy and productive, but the loss of old-growth trees should not go unnoticed. The giant trees have unique traits that support a number of animal and plant species and make for a diverse, robust ecosystem. In a similar sense, the same is true for old fish.

The designation of an "old fish" varies from species to species, depending on life history. Some types of rockfish might live to 200 years, while few herring live past age 10.

After female fish release eggs, many factors must align for a healthy brood to hatch and grow to adult size. Because the marine environment is so variable, species might go a whole decade between successful broods. Older fish in a population have more years to produce eggs, increasing the chance for success over time.

"In the marine world, the success rate of producing new baby fish is extremely variable," said co-author Trevor Branch, a UW associate professor of aquatic and fishery sciences. "I think of old fish as an insurance policy -- they get you through those periods of bad reproduction by consistently producing eggs."

In addition to having more opportunities to reproduce, older fish also behave differently than younger fish. As they age, some fish change what they eat and where they live in the ocean. They also take on different roles in the marine food web, sometimes becoming a more dominant predator as they get older, and bigger.

The researchers looked at model output gathered from commercial and recreational fisheries and scientific observations that describe the status of fish populations over the years. In their analysis of 63 populations living in five ocean regions worldwide, they found that the proportion of fish in the oldest age classes has declined significantly in 79 to 97% of populations, compared with historical fishing trends or unfished figures, respectively. The magnitude of decline was greater than 90% in 32 to 41% of the groups.

This is mainly due to fishing pressure, the researchers say. In general, the longer a fish lives, the more encounters it has with fishing gear, and the greater the likelihood it will be caught. However, some environmental factors like disease and pollution might also contribute to the loss of old fish.

These findings could inform fisheries management, which often sets limitations based on the total weight of fish caught over a season without considering factors such as the size or age of a fish. The authors suggest fishing methods to protect young and old fish by prohibiting the harvest of fish below and above a specific size range. Other solutions include closing certain areas to fishing permanently, or rotating areas where fishing can take place each year to let fish grow older and bigger -- similar to agricultural crop rotations that allow the soil to recover between planting cycles.

The title of the blog refers obviously to great Cohen brothers movie "No Country for Old Man"

Journal Reference:

  1. Lewis A.K. Barnett, Trevor A. Branch, R. Anthony Ranasinghe, Timothy E. Essington. Old-Growth Fishes Become Scarce under FishingCurrent Biology, 2017; DOI: 10.1016/j.cub.2017.07.069