(1) A (2) B
(3) C (4) D

Pelagic Fish Survivorship Curve: Which Type Do They Exhibit and Why?

Understanding the survivorship curves of different organisms is essential in ecology, as these curves reveal critical information about the life history and population dynamics of species. Among aquatic organisms, pelagic fishes—those that inhabit the open ocean and upper water column—are particularly interesting due to their unique ecological strategies. This article explores the type of survivorship curve observed in pelagic fishes, the biological reasons behind this pattern, and its implications for marine ecosystems.

Introduction to Survivorship Curves

Survivorship curves are graphs that plot the proportion of individuals in a cohort (a group born at the same time) that survive from birth to each subsequent age. These curves are broadly classified into three main types, often referred to as Type I, Type II, and Type III, but sometimes labeled as A, B, and C or I, II, and III depending on the convention used.

• Type I (A): Low mortality in early and middle life, with most deaths occurring in old age. Characteristic of large mammals and humans.

• Type II (B): Constant mortality rate throughout life. Seen in some birds, rodents, and invertebrates.

• Type III (C): High mortality in early life, but those that survive to adulthood tend to live longer. Typical of many fish, insects, and plants.

What Are Pelagic Fishes?

Pelagic fishes are species that live in the pelagic zone—the open ocean, away from the bottom or the shore. Examples include herring, mackerel, tuna, and anchovy. These fishes are adapted to life in large, open water environments and often form large schools. Their life history is shaped by the challenges of predation, food availability, and the vastness of their habitat.

Why Do Pelagic Fishes Exhibit a Type III Survivorship Curve?

Pelagic fishes are classic examples of species with a Type III (C) survivorship curve. This means that they experience very high mortality rates during their early life stages, especially as eggs and larvae. Only a small fraction of offspring survive to reach adulthood.

Biological Basis of Type III Survivorship in Pelagic Fishes

1. High Fecundity:
   Pelagic fishes typically produce a vast number of eggs—sometimes millions per female per spawning event. This reproductive strategy is a response to the high mortality risk faced by offspring.

2. Low Early Survival:
   Eggs and larvae are vulnerable to predation, starvation, and environmental fluctuations. The open ocean is a challenging environment for small, defenseless young.

3. Increased Adult Survival:
   Those individuals that do survive to adulthood have a much higher chance of living longer, as they are larger, more mobile, and less vulnerable to predators.

Ecological Implications

The Type III survivorship curve is a hallmark of r-selected species—those that maximize reproductive output to compensate for high juvenile mortality. This strategy is common in environments where resources are unpredictable, predation is intense, and competition for space is less of a limiting factor.

Visualizing Survivorship Curves

A survivorship curve for pelagic fishes would show a steep drop in the number of survivors during the earliest life stages, followed by a much flatter curve as the remaining individuals reach adulthood. This is the classic concave (Type III) curve.

Comparing Survivorship Curves

Why Not Type I or Type II?

• Type I (A):

  • Characterized by high parental investment and low juvenile mortality.

  • Not seen in pelagic fishes, as they do not provide parental care and face high early mortality.

• Type II (B):

  • Indicates a constant risk of death at all ages.

  • Rare in pelagic fishes, as their mortality is heavily skewed toward early life stages.

Life History Traits of Pelagic Fishes

Pelagic fishes exhibit several life history traits that are consistent with a Type III survivorship curve:

• High reproductive output:

  • Compensates for high juvenile mortality.

• Low parental investment:

  • Eggs and larvae are left to fend for themselves.

• Short generation time:

  • Allows for rapid population recovery.

• Early maturity:

  • Enables individuals to reproduce before they are likely to die.

Population Dynamics and Survivorship

The high early mortality of pelagic fishes means that their populations are highly sensitive to changes in the survival of eggs and larvae. Environmental factors such as temperature, salinity, and food availability can dramatically affect the number of individuals that reach adulthood. This sensitivity makes pelagic fish populations highly dynamic and variable over time.

Conservation and Management Implications

Understanding the survivorship curve of pelagic fishes is crucial for their conservation and management. Because so few individuals survive to adulthood, protecting spawning grounds and ensuring healthy larval environments are essential for maintaining sustainable populations. Overfishing of adult stocks can have severe consequences, as the loss of even a small number of adults can significantly reduce future recruitment.

Case Studies: Pelagic Fish Survivorship

Atlantic Herring (Clupea harengus)

Atlantic herring is a classic pelagic fish with a Type III survivorship curve. Each female can produce tens of thousands of eggs, but only a tiny fraction survive to adulthood. The survival of larvae is influenced by ocean currents, temperature, and the availability of planktonic food.

Pacific Sardine (Sardinops sagax)

Pacific sardine populations are also characterized by high juvenile mortality and a Type III curve. The genetic diversity of these populations is shaped by the large variance in reproductive success and the high mortality of early life stages.

The Role of Environmental Variability

Environmental variability plays a significant role in the survivorship of pelagic fishes. Fluctuations in temperature, salinity, and food availability can lead to dramatic changes in larval survival rates. This variability is a key reason why pelagic fish populations can experience large swings in abundance from year to year.

Survivorship Curves and Evolutionary Strategies

The Type III survivorship curve is an evolutionary response to the challenges of the pelagic environment. By producing large numbers of offspring, pelagic fishes increase the chances that at least some will survive to adulthood, even under highly variable conditions. This strategy is common among organisms that inhabit unpredictable or hazardous environments.

Summary Table: Survivorship Curves in Fishes

Conclusion

Pelagic fishes exhibit a Type III (C) survivorship curve, characterized by high mortality in early life stages and much lower mortality among adults. This pattern is a result of their life history strategy, which emphasizes high fecundity and low parental investment to cope with the challenges of the open ocean. Understanding this curve is essential for the conservation and management of pelagic fish populations, as it highlights the critical importance of early life stages in maintaining healthy and sustainable stocks.

Answer to the Question:
The type of survivorship curve observed in pelagic fishes is Type III (C).
So, if the options are labeled as:
(1) A (2) B (3) C (4) D
The correct answer is (3) C.