Researchers from the National Oceanic and Atmospheric Administration (NOAA) have discovered why thousands of sea lion pups are starving.

The culprit behind the saddening discoveries of emaciated and starving sea lion pups is the poor quality of food that breeding females are able to forage.

The resulting effect is that the juvenile sea lions do not get the adequate nourishment they need, explaining the tremendous increase of the species needing help from various animal centers.

Increasing Population, Decreasing Weight

While the sea lions' population in California has shown a significant rise from 50,000 to 340,000 over the last four decades, the trend of the species' weight tends to go toward the opposite direction. Aside from that, the pups have increasingly been found stranded and starving in the southern part of California.

Over the years, young sea lions have seen recurring trends of having thinner than normal bodies and more fatalities than usual. Such events were noted in 1983, 1992-1993, 1997-1998, 2009 and 2013.

Although experts believe that environmental factors, such as toxic marine algae, have significant impacts on these events, the exact mechanisms behind are not clearly understood and are recognized to possibly differ in the each event.

Further into the past, different industries massively took advantage of sea lions in California (Zalophus californianus) due to their meat, oils and hides. After obtaining such goods, the species had been killed. This had been the trend from the latter part of the 1800s. Up until the early 1970s, the populations of the said species have remained low.

It was only in 1972 when the population of sea lions rose after the Marine Mammal Protection Act (MMPA) was implemented. The number of species shot up swiftly, maintaining an annual rate increase of more than 5 percent over the past 36 years in the Channel Islands of California.

Foraging Of Sea Lions

Sea lions forage on a variety of marine items. The species' diet changes within a year, but the most usual items on their list are market squid, northern anchovies, Pacific sardines, shortbelly rockfish, young Pacific hake, Pacific mackerel and jack mackerel.

The amount and type of fish consumed by the sea lions vary depending on the region, season and environmental state.

Sea lions generally pattern their diet to the current condition, such as in times of scarcity. The species are opportunistic foragers as they can switch from one food to another and increase the diversity of what they can eat.

During El Niño, sea lions settle for less nutritious and less favored food such as young Pacific hake and rockfish, in replacement of the heavily preferred anchovies and sardines.

One instance of this adaptability is when sea lions were found to allot a significant portion of market squid in their diets, even if the said food item is recognized to be less preferred.

Low Calorie Diet

The NOAA study, published in the journal Royal Society Open Science, investigated the supply of the sea lions' four preys, which are anchovies, sardines, racket fish and market squid.

Sardines and anchovies are high in fat and very essential to growing juvenile sea lions. In the study, the researchers found that the populations of these preys plummeted since the middle part of 2000s in regions near Channel Islands where female sea lions feed.

With this, the mother sea lions were pushed to eat the low-fat, low-calorie rockfish and market squid instead.

Such trend in the availability of food supply has vital effects to the quality of food that female sea lions are breeding at the rookeries. Scientists describe this as nature subjecting sea lions to low-calorie diets.

While adult sea lions may survive with such diet, pups and lactating mothers cannot. In fact, they seem to be more sensitive to cuts in their high-calorie preys.

Environmental Shifts

The nature of extensive prey modification over a long period of time suggests that it is caused by environmental shifts, the authors of the study say.

The researchers explain that the foraging trends are mostly environmental but the effects of humans and climate cannot be disregarded either.

In the end, they say more malnourished and starving sea lion pups may be expected in the coming years. However, forecast fluctuations make the entire picture bleak.

"We cannot predict how long low forage abundance may last," the authors write.